Ghast

Ghasts are very large jellyfish-like mobs that float through the air and shoot fireballs from their mouths. Their bodies are 4 by 4 by 4 block cubes with 9 tentacle-like limbs hanging from the bottom. They first appeared in the Halloween Update. Ghasts are able to spawn anywhere in the Nether, as long as there is sufficient space (5*5*5 space is required to spawn) and the game is not in Peaceful mode.

Behavior
Ghasts normally float around with their eyes and mouth closed and periodically make high-pitched, child-like sounds. They open their eyes and mouth when attacking. They make a chirp-like noise when shooting and scream loudly when taking damage. The sounds that Ghasts make have a minimum intensity regardless of their distance, so they often sound closer when they are actually far away. As of Beta 1.8.1, the Ghast will let out a death rattle upon being killed - similar to the death cries that Zombies and Spiders use when killed.Fireballs do not home in and can be deflected by the player,so fireballs are easy to avoid if the player knows that it is coming.

Unlike most other aggressive mobs, the Ghast does not have a pursuit mode - it does not attempt to draw closer to the player when at long range (and likewise is not particularly likely to float away). Compared to other mobs, the Ghast has a very long search range and can target a player up to 100 blocks away. When the player comes within about sixteen blocks, the Ghast will open range and attempt to gain altitude.

Ghasts will not fire at the player without a line-of-sight - this can be blocked by Glass, Glass Panes, Leaves, Nether Portals, Vines, Iron Bars, both types of Fences, and Cobwebs - making safe scenic paths through the Nether a viable possibility. When not moving, Ghasts within firing range will face the player, giving the impression that they can see through transparent materials. Ghasts can draw a line-of-sight through curtains of lava, and their fireballs can pass through it meaning players near a pillar of lava can be struck without seeing the source. However, the Ghast can "cancel" its fireball while shooting if the player moves behind an obstruction or builds one.

Ghasts may drop a Ghast Tear when killed, but this can be hard to collect - because it is a flying mob, the tear may fall into lava or an inaccessible area. Ghast tears are a positive potion ingredient, for many useful potions, so Ghast tears are well worth obtaining.

If shot at by a Ghast just try to hit the fireball back at the ghast, if you happen to hit the ghast it will die in one hit.

Ghast Fireball
The Ghast's fireball attack has a straight trajectory unaffected by gravity and an unlimited or very long range limited in practice only by the confines of the Nether's caverns. Their explosions can often be heard without the impact being seen. At close range, it is the most powerful mob attack in Minecraft, along with the blast of a Creeper. A direct hit from point blank range will deal a full 10 hearts of damage but damage decreases with distance and is less if the player is simply caught in the blast radius. Additionally, the projectile is fairly slow and aims slightly above the player's head. When two Ghast fireballs collide, one is deflected to a random direction while the other explodes upon impact. Hitting a Ghast fireball with an item held in hand, an empty hand or a fired arrow will deflect the fireball back in the direction it came. Prior to Beta 1.9 Pre-releases, it looked like a flaming snowball. Now it uses the same texture as a Blaze fireball.

If a Ghast's fireball hits a portal or hits close to it the portal will be closed, requiring re-ignition. If the player manages to get a Ghast to fire at the portal while it is closed, it can be reignited using the fireball. Blocks originally ignited once struck, adding an extra hazard to fighting Ghasts; however, the fire is extinguished immediately afterwards because of a bug (fixed as of 1.9 pre 4.)

The minimum block resistance required to absorb all blast force of the Ghast's fireballs is 20.17, meaning that fences and other blocks with low blast resistance will be destroyed in a fireball explosion.

Fighting Ghasts
Ghasts are often considered the most difficult mob to kill in the nether owing to their extreme attack range, explosive attack power, mobility and flight. They are also quite common; players in any exposed area will have to deal with them sooner or later and possibly from the moment they step out of a portal. Ghast fireballs have a profound effect on Nether architecture, making a compelling case to use only materials resistant to their explosions for outer walls and exterior fittings in places where their fire may be drawn. As a result of their large size and the extreme unpleasantness of an indoor Ghast spawn, the primary defense mechanism for buildings in the Nether becomes enclosure rather than lighting. A Ghast's firepower is highly inaccurate at most ranges if one is moving. Players are most vulnerable when they are building in the Nether, when they may well not be moving and possibly standing above a long fall. Ghasts can also break blocks from under the player, often dropping them into lava or causing death through falling damage.

If a Ghast is trapped in a small room, the player can make a defensive wall, make the Ghast shoot, and the fireball will kill the Ghast.

The player can reflect fireballs by hitting them with anything before they impact, even bare hands. Because of their low speed, spamming mouseclicks is often enough to deflect any given attack. It is possible to kill Ghasts by reflecting their attacks back at them, but it can be difficult since ghasts are agile and their hitbox is restricted to their tentacles. A reflected fireball is able to kill an unhurt ghast in one hit. You can make a fireball easily reflect in the direction you want by shooting an arrow, fishing rod, or snowball at the fireball. The fishing rod, when released, slightly affects the fireball's trajectory. This relationship goes both ways and a Ghast's constant stream of fireballs may well interfere with the firing arc of your bow.

Most Ghast kills are made with a Bow and Arrows for obvious reasons but a Ghast can be killed with melee weapons if the player can get close enough before it draws away. Ghasts are highly vulnerable when caught on the ground since their attacks can be deflected and their health is quite low - 2 blows from a diamond sword will destroy one. It is possible to use a Fishing Rod to draw Ghasts near enough to hit them with a Sword. A close Ghast is also much easier to hit with a bow, though reaction time against its fireballs is likewise lessened. When aiming your bow, be sure to lead your shots as Ghasts can quickly change trajectory and move quite quickly. Depending on the Ghast's range and relative altitude, it is usually necessary to also aim at a high trajectory to gain extra range. Where possible, having the advantage in altitude (such as shooting from the topmost layer of the Nether) is of enormous help. If ammunition is not a concern, it is advisable to shoot many arrows at differing trajectories in order to find the range to the Ghast. The Ghast's hitbox is mainly its tentacles, and aiming too high may cause a shot to go through the Ghast's body, dealing no damage. However, it is perfectly practical to shoot the hitbox through the ethereal body from above using plunging fire.

Much like using neutral or enemy mobs as shields against arrows fired by skeletons in the regular world, it is feasible to use Zombie Pigmen as shields against Ghasts in the Nether. It is possible to injure or kill zombie pigmen in this fashion without ever making them hostile to the player.

An effective method of fighting Ghasts in the nether is to make a shelter, preferably out of brick, cobblestone, obsidian, or other explosion-resistant materials. Make a 1x1 hole in the roof and add ladders leading up to it, in such a way that you can poke the upper half of your body through the roof. When a Ghast is near, climb the ladder, shoot an arrow at the Ghast and quickly descend again into your explosion-proof shelter. This should mean that you can kill Ghasts easily with minimal health loss. To improve this even further, add a small 3x3 roof 1 block above the hole in the main roof. Most of the Ghasts' fireballs will now hit this roof rather than you, meaning that you should be safe against most Ghast attacks.

To solve the issue of drops being impossible to reach, build a large floor to kill the Ghasts over. With a lot of extra time and cobblestone, a large structure with many floors, each tall enough for Ghasts to spawn inside of, can be built. This makes them very easy to farm, though it is very time and material intensive to make such a structure large enough for them to reliably spawn inside of.

Since the Beta 1.9 Pre-release 3, Ghasts do full 10 hearts damage when a direct hit from a Fireball is taken. Inversely, this means that deflecting a Ghast's fireball against itself will kill it instantly.

It will take a couple shots to kill a Ghast with a Bow or 2 hits from a diamond sword.

Resistance to Fire
Ghasts may also be set on fire, but Fire and Lava don't actually damage them. A similar effect is seen with Zombie Pigmen. When a Ghast is caught in lava, its movement is impeded, and can then be easily targeted with arrows. At around 23:09 in this video, a Ghast impeded by a lavafall can be seen, and again, but closer and for a longer period of time, at 23:59.

It is disputed that Ghasts can be killed by lava or fire if continually re-exposed. This is achieved by a Ghast floating in and out of lava. There have been reports of zombie pigmen dying while in lava, due to the fact they "bob" in and out of lava,(a rumor about zombie pigman is that in the lava they gain fall damage which affects them when they exit the pool of lava.) which raises doubt that Ghasts and zombie pigmen are completely lava/fireproof.

History

 * Ghasts were introduced in Alpha 1.2.0
 * Ghasts and Zombie Pigman were the first mobs introduced to the Nether.
 * Ghasts used to be vulnerable to fire and lava, but since the Alpha 1.2.2 update, this bug has been fixed and they no longer get hurt when they come in contact with lava or fire.
 * Prior to Beta 1.7 Ghasts aimed at the camera, not the player. Pressing F5 allowed one to easily send the fireball over their head instead of at the character.
 * Ghasts used to not be affected by light, and appeared with the same brightness anywhere. This was changed in Beta 1.9 Pre-release.
 * Since the Beta 1.8, Ghasts are more easily killed than in previous versions, due to the reworking of bow mechanics that came with the update.
 * Since 1.9, Ghasts are one of the few mobs that will not turn aggressive in creative mode, even when you hit it.
 * Since 1.9, Ghasts are able to be killed by their own fireballs, allowing the achievement "Return to Sender."
 * Prior to 1.9 you could only injure Ghasts by hitting their tentacles, as their hit box was not large enough to incorporate the body.
 * Prior to 1.0.0, Ghasts used to have a snowball-esque fireball texture. In 1.0.0, the ideal "fireball" texture is now used.

Ghosts
For any of you who were re-directed here looking for ghosts: A ghost was not a mob but simply a glitch (that is the most popular conclusion) but it was never specified by Notch or anyone else from Mojang but many people think it was the fixing of a glitch that happened at night were, on peaceful, hostile mobs would spawn for a split second before vanishing. You can read more about it Here.

Trivia

 * Contrary to popular belief, Ghasts will spawn on blocks. Therefore they can't spawn on transparent blocks no matter how high the roof is. If any transparent blocks protrude into their spawning area (5x5) they cannot spawn.
 * As of 1.1 in .minecraft/resource/newsound/mob/ghast, ghasts have an affectionate scream, which is a high pitched horn sound (like the classic cartoon tugboat sounds).
 * In Creative Mode, you can go inside of a Ghast by falling through the top of it.
 * Originally, Ghasts were intended to be able to spawn near Nether Portals in the Overworld. However, this ability has not yet been implemented.
 * Despite being fireproof, Ghasts and Zombie Pigmen are capable of drowning if under water or lava for too long. This means they, like the player and other mobs, require air to live.
 * Ghasts can cause other mobs to attack it, but the Ghast will never target other mobs; it will only target the Player.
 * It is possible to kill Zombie Pigmen by deflecting Ghasts' fireballs at them. It is also possible to deal a Critical hit on a fireball, since it is an entity.
 * Prior to the full release of Minecraft, when a Ghast shot a fireball, it looked like a flaming snowball; it has since been updated with a new magma stone texture.
 * Ghasts are the largest mob by volume - they are eight times as big as the largest variation of slime, but Enderdragons are longer.
 * It is impossible to 'ride' a Ghast as the player will fall right through it.
 * If a Ghast is trapped in a cube, it will shoot fireballs through the cube. This is because like all other things that shoot, it spawns the projectile one block in front of it.
 * Due to the explosive property of Ghast fireballs, they can be used to mine blocks in the Nether.
 * Ghasts (like all other flying mobs, including the Player in creative mode) can actually fly much higher than y-coordinate 128.
 * Using a tool to spawn a mob riding a ghast will cause the mob to be inside the ghast, unless it's a giant.
 * Ghasts take damage from touching cacti.* Similarly, for Ghasts on fire, the flame texture will enlarge on the Ghast.
 * Some rooms in Nether Fortresses are big enough for a Ghast to spawn in. However, this Ghast will likely be trapped, as there may not be an exit large enough for it to fit through.
 * A Ghast's eyes appear to be full of tears, it is unknown why they are constantly crying/sobbing.
 * Ghasts, even though they can fly, will not spawn above water if it is hacked into the Nether.
 * If a Ghast is killed, any Fireball the Ghast has launched will disappear.
 * Their fireballs can destroy Block 97, but this would kill the silverfish inside of the stone block instead of spawning it.
 * In 1.1, Spawn Eggs are the only way to see Ghasts in the Overworld without hacks.
 * If you tweak a Mob Spawner to spawn Ghasts, the model inside the spawner is larger than the frame.
 * Ghasts appear to be 4x4x8 blocks high, but since their hitbox only covers the lower half of their body, they are technically 4x4x4 blocks
 * A well timed arrow can knock back the projectile into the ghast.

Bugs

 * Sometimes, if there is a 1-4 space between a platform, they will be trapped, but will still be able to fire at you.
 * Their hitbox data seems out of place. Objects will pass through the top of a Ghast, however, hitting its tentacles will hurt it.
 * Arrows will sometimes pass through the Ghast during server lag in SMP.
 * In SMP, their fireballs often move back and forth, similar to arrows. This is due to lag.
 * If spawned in the Overworld (by using hacks) in a glass cage, they will float and stare at the player, but Ghasts will sometimes unexpectedly shoot a fireball in the direction of the player even though there is a glass barrier between them and a player. This bug appears to be absent in the Nether.
 * A fireball will not disappear if in flight when switching to peaceful.
 * Ghasts' explosions are less "connected" to them in SMP than in SSP. For this reason in SMP mobs damaged by a Ghast's fireballs will not attempt to attack the Ghast and Ghasts that are either spawned or follow a player into the surface world (see below) can become divorced from their explosions and become an essentially peaceful mob. Similarly, Ghast explosions can sometimes follow a player into the real world, without any nearby ghasts, but these normally only appear in the form of apparently random explosion noises not connected to actual damage causing explosions.
 * When a player in an SMP server dies near Ghasts or is teleported out of the Nether to the surface world (not via a portal) there is a chance the Ghasts will follow them to their new location in the surface world. These Ghasts may act like normal Ghasts but could also behave radically differently than normal. Changes in teleported Ghasts include the Ghast-less explosions and Ghasts without explosions mentioned above. The teleported Ghast may also freeze in one location or "stalk" a certain player (moving closer to a player until they touch like a normal hostile mob) but both of these are extremely rare.
 * Arrows used to look bigger when they were stuck on Ghasts but as of beta 1.9 pre release this no longer happens.
 * Similarly, when a Ghast is on fire, the flame texture will be enlarged. This however, also happening to spiders and other mobs, is realistic and not a bug.
 * It is possible to deflect a ghasts fireball with a fishing hook, which will cause your line to move with the deflected fireball.
 * Sometimes, when the ghast is hit by an arrow (at full power/draw) at long range or while not in combat it will not die, though the arrow will still appear to become stuck in the ghast.
 * Sometimes, attacking a Ghast may not make it attack you when you are at a range of 16 blocks or less.
 * If you get too close and touch the ghast, it is so big that you will see yourself inside the ghast, and the body wall and the stubs of tentacles sticking out of the bottom.
 * In SMP, after being bumped by a passive mob in the Nether (especially whilst standing in a waterless chicken farm), Ghasts have the ability to target the player through walls and shoot fireball after fireball, creating a tunnel to the player, killing said player. Ghast continues to fire until the player respawns, which can result in anything flammable burning, chests exploding and more. Recreated twice, 20 blocks underground.