Hazards

Hazards are common dangers or things that can cause injury/death.

Mobs
Found at all heights from the base of the world, although most common at water level. [citation needed]

Best way to counter the danger is to use walls to block their movement (and attacks in the case of skeletons), use armor to absorb some/most damage, be well-armed, and survey surroundings carefully.

Note: Forests and jungles have some advantages, such as "cover" from skeleton arrows, and obstacles to delay monsters. The flip side is that the player can also be caught between trees (usually leaves), and monsters can hide behind the trees. At the other extreme is desert, where you can hop onto a hill and see every monster within render distance.

Here is a summary of the common mobs that come out at night (and underground). There are others that can appear in particular places, but these five can be found anywhere dark.
 * Zombie: Weak. When it sees you it will walk to you and hurt you if it touches you. An easy way to kill one is to wait until it is in the reach of your sword, and then hit it. The Zombie will fly back a little, and you can wait until it gets near again and hit it again. Since 1.2.1 zombies actually have good AI and can run around corners and walls. There are your ideal first kill.
 * Skeleton: Medium. Skeletons will shoot at you when they see you, and they can't run out of arrows. They are hard to kill with a Sword, so you should probably choose another mob for this achievement. They are, however, very easy to kill with a Bow.
 * Spider: Medium - weak. Spiders will attack you at night, but won't attack you if you don't hurt them during day. They can be killed fairly easily if you hit one, run backwards and hit it again when it jumps. You may get hurt, but you will probably manage to kill the Spider.
 * Creeper: Strong. Don't try to kill it without practice or a Bow. Creepers will walk towards you when they see you and explode if they come near. They can be killed with a Bow easily, and with a Sword by hitting it and running backwards until it stops glowing (so it won't explode). Repeat until you kill it. Cats will drive creepers away (usually).
 * With the addition of sprinting, it is much easier to fight creepers with a sword. Hitting it while sprinting will knock the creeper back far enough to cancel its attack. However, be especially careful, as if the creeper's flight is interrupted by a block in its path it may still be in range.
 * If you want to cheat a bit with Creepers, set the difficulty to Easy. Hitting a creeper with a sword and running back 5 blocks will immediately cancel the explosion.
 * Enderman: Strong. Endermen won't attack you unless you look at them or attack them. They are quite strong in terms of damage and health, but they can teleport around, and they can easily catch you off guard when they attack you from behind. They cannot be damaged from projectiles, so bows are useless against them. Leading them into water will get rid of them; if you want to hunt them, build a 3&times;3 or bigger ceiling 2 blocks above level ground, and wait under there for the monsters to come out (watch for anything coming up behind you). When you spot an Enderman, stare at it, and whack it while the 3-block-tall monster is stymied by your 2-block-high ceiling.

Water
Found at all altitudes but in greatest quantities around the main landmass.

Danger is from drowning - any time there are two water blocks in depth there is the risk of drowning.

Best way to counter the danger is to move around the water rather than through it, to keep jumping which makes the player swim on the surface or to build a bridge through the water. Ponds may be drained with buckets or filled in.

Lava
Found usually below water level, and especially at level 10, where huge lava lakes can appear.

Danger is from contact - players are set on fire and stay alight until they jump in water, die, or the fire burns itself out. Lava pockets are often difficult to detect before being tunneled into, although as long as you do not mine directly up or down you will usually be able to outrun the lava flow.

Note: Using water the lava can be transformed into harmless obsidian - players should be wary of digging through or around either of these in case the lava is more than one block deep.

Gravel/Sand
Gravel: Found at all altitudes

Sand: Found sometimes near water and in the desert.

Gravel and sand are the only two blocks affected by gravity. As such, if a player manages to have two or more blocks of gravel or sand fall on top of them, they will suffocate.

Falling
Falling damage can be caused by jumping of a cliff (whether it be deliberately or accidentally) it can cause many damage. Fall damage can be calculated using this formula: FD = (number of blocks - 3) / 2 It can be avoided if
 * Sneaking (default ) will prevent you from falling off most blocks.
 * Having some amount of Enchanted Armor may save your life from otherwise fatal falls.
 * Landing in water will reduce fall damage. Depending on the height of the fall, it needs up to 3 blocks of water to reliably nullify the fall damage; however, even 1 or 2 blocks may sometimes do so.
 * Moving into a ladder's area of effect during freefall will reduce vertical velocity to normal ladder descent speed and no damage will be taken. Care must be taken to avoid landing on the narrow top surface of the ladder.
 * Moving into water during freefall will also reduce velocity.
 * Move passive mobs away to avoid an unpleasant push to your death.
 * You might not sustain any fall damage while you're on fire.
 * Jumping counts as falling for 1½ blocks, so keep this in mind when trying to avoid fall damage.
 * Do not dig directly downwards without knowing what is underneath. Empty caverns are randomly placed around the world, and it's quite possible to fall into them (perhaps from a great height, or even onto a Dungeon or Lava.).
 * Don't attempt to jump gaps across long drops. Instead, sneak to the edge of your current platform, and you will be able to place a block beside it; in this way, bridges can be constructed safely.
 * With good timing, you can place a block on the ground under you while falling. This will only reduce damage by a little, but will come in handy if you need to fall 4 blocks, as it will prevent all damage. If a vertical surface is available, a block can be placed on that as well with practice.

Lightning
Lightning occurs during storms, and deals initial damage as well as setting the player on fire. Although it is rare, it can be extremely deadly, so it is recommended to either sleep in a bed or stay inside during storms.

TNT
Only man-made, although if placed/detonated incorrectly is incredibly dangerous.

Danger comes from both the explosion and the resulting crater, which can deposit players in deep caves, causing fall damage.

Other hazards
Dungeons can be particularly dangerous - bursting into one by accident with TNT can spell not only a single death but also a mob-ridden section of your base. Most people consider dungeons worth raiding however, because if lucky you can get stocks of Iron, Redstone Dust, Gunpowder, Saddles (which are uncraftable), Apples, Cocoa Beans, and Music Discs, which are otherwise difficult to obtain.