Lava

Lava is a fluid block that can only be placed by the player when using a bucket. It emits light and sets surrounding flammable blocks (wood, fences, etc.) on fire. On newly-generated maps, lava is a common sight underground and in lava lakes that occasionally generate on the surface or most likely found anywhere at the floor of the Nether.

Natural occurrence
Lava primarily occurs as magma in large pools deep underground, on levels 1 through 10, covering 8% of horizontal area. It can also occur as lava rivers from a single spring block, pouring down walls into pools. It is one of the most common causes of death while mining underground and is especially nasty because the items in your inventory are likely to burn up in the lava. But, if precautions are taken (not digging straight down or up, having buckets of water), accidents can be prevented. The popping sound effect of lava added in version 1.2 can also be an early indication of nearby lava.

Since Alpha 1.2.6, lava also spawns as lakes. They are rare and can be found at any elevation within any biome, can cause forest fires, and sometimes have floating chunks of stone or dirt above them, occasionally with vines or trees growing on the floating chunks (very small islands are sometimes in the center of a lava lake as well.) Lava lakes vary in size and depth, and the lakes will carve out a small ceiling area above them when generated below the surface.

2 blocks of lava can also be found in NPC Villages in Blacksmiths' houses.

In the Nether, lava is extremely common, appearing more frequently than water in the Overworld. Lava also pours from the ceiling and walls. As of the 1.5 update, there are randomly placed single lava blocks hidden inside the netherrack. This makes tunnelling through netherrack very risky.

In the Extreme Hills biome it flows down sometimes an overhang making it easy to get obsidian if a water source is nearby.

Burning

 * Most entities, including players and mobs, will take damage every half-second from being in contact with lava, and will also be set on fire.  Nether mobs (which are immune to fire) will take no damage, nor will players or mobs affected by a potion of fire resistance.  In the Overworld, lava is extremely deadly and kills more or less anything in its wake.  If the victim touches water or rain falls on it, the fire will be extinguished, but the lava will continue to damage them directly.
 * TNT, bookshelves, leaves, wool, fences (but not gates), vines, tall grass, wood logs, planks, and wooden stairs are flammable. Wooden slabs and double slabs created in 1.3 and above are flammable; ones created in 1.2.5 or below are not, even if used in 1.3 or above, as they are based off the stone slab block. If flammable blocks are close to lava they can catch on fire, the mechanics for this are the same as of fire. Non-flammable blocks are unaffected by this effect, and do not spread active fire.
 * The “embers” or “fireballs” which fly out of lava are purely decorative and do not cause fires or damage to entities.
 * Warning: even if you are playing on peaceful difficulty, it is still possible to die. The regenerating hearts and loss of hearts from being on fire will simply counteract each other, and the player's hearts will fluctuate by only a half a heart, keeping them alive and giving them longer to find water. However, if you stay in the lava itself, you will lose hearts much more rapidly and die.

Flow


Like water, lava flows from "source blocks". Most streams or "lava-falls" come from a single source block, but lava lakes (including the "flood lava" in the bottom 10 layers) are composed entirely of source blocks. Only a source block can be captured with a bucket. Lava flows far more slowly than water, and sometimes sourceless lava flows will linger for a very long time. Formerly, lava flows could remain for a very long time (even hours) without their source block, but this bug was fixed in version 1.6. In the Overworld, lava travels 3 blocks in any direction from a source block, but if it is placed above the ground, it will travel 4 blocks in any direction after hitting the ground. In the Nether, lava travels 7 blocks in any direction as water does in the Overworld. In the Overworld, lava flows at a rate of 1 block every 1.5 seconds, or 0.6 blocks per second, although as of the 1.5 update it flows more quickly in the Nether.

Lava which is flowing will destroy the following in its path: saplings, cobweb, tall grass, dead bush, wheat, flowers, mushrooms, snow on ground (but snow blocks are immune), lily pads, vines, levers, buttons, both types of torches, redstone wire, repeaters, and rails. Sugar cane holds back lava, but will disappear if the sugar cane's water source is destroyed by the lava.

Using a redstone wire, a one-block lava flow can be redirected by supplying power to the spring block, which will cause it to reset the flow towards the now-nearest terrain depression. This is further elaborated in this thread (only viewable when logged-in). It cannot, however, be reversed. This re-calculation is made because redstone wire when toggled changes the block from redstone(on), to redstone(off). Whenever a block updates on any side of lava, the lava re-calculates where to flow, but does not cut off its current direction of flow.

Lava and water

 * If lava flows on top of still or running water, it creates stone.
 * If lava flows horizontally into water, cobblestone is created. Using this you can create cobblestone farms.
 * If water flows horizontally into lava, a hiss and puff of smoke occur but nothing changes.
 * If water flows vertically into lava, cobblestone or a hiss may result.
 * If water flows into a lava source block then obsidian is created. The lava spring is destroyed in the process, so unlike cobblestone it is not continuous and renewable.
 * If vertically falling water touches a lava source block on any side obsidian is created - even if the water would not run into the lava's square.

Other points


If lava falls onto redstone wire bordered by water, the redstone turns into obsidian, with no loss of lava. This is considered a bug, however Notch has said that it will remain for the meanwhile as a feature; see the section on obsidian for more information.

If there is a body of lava flowing above a block, the lava visually seeps down through the host block. This "dripping" effect also applies to water, although water droplets seep through at a much faster rate. However this does not occur with transparent blocks, because the game assumes that you can see through them, which is a danger when the lava is above stairs, slabs, or other nominally transparent blocks. These droplets are purely aesthetic and will not affect the player or other blocks in any way.

Lava Bucket
You can collect and place lava springs using a bucket. As with water, you can only harvest the springs, not the flowing lava coming from them.

Lava can be used to make obsidian, to light an area, or to create traps. When used to create obsidian, water will be used to cool the lava either before collecting it with a diamond pickaxe in a mold in the desired shape, or by emptying the bucket next to an artificially created waterfall.

Lava is a very efficient fuel. It has the longest burning value of 1000 seconds, compared to 80 seconds for coal (lava smelts 100 blocks, coal smelts 8). After all items have been smelted, an empty bucket remains, except sometimes on Minecraft: Xbox 360 Edition, the bucket disappears.

Lava buckets are often used as weapons due to its powerful damage and re-usability. Its use as a weapon is documented in the combat tutorial article.

Lava molding
Lava can be combined with water for molding obsidian, stone or cobblestone structures of any size when used properly:

Pros

 * One does not need to contain lava on all sides; lava flowing freely down is often used.
 * The size of structures is almost limitless (the sky IS the limit). Many structures are massive.
 * Lava molding can have very desirable architecture, mostly from downward flowing lava.
 * With only a few starting blocks, one can build very massive structures.
 * Lava molding is much faster than building by hand (this is limited to larger structures).

Cons

 * Lava molding can be very dangerous to players and other entities alike.
 * If one is not careful, they may solidify the source block, creating obsidian.
 * One is often limited to walls and towers.
 * Structures can only be made from either stone (which is harder to get), obsidian, or cobblestone.
 * If you have a full inventory while you fill up more buckets of lava, the filled buckets that don't fit in the inventory will drop out like when you click Q.

Bugs

 * In taiga biomes, lava will occasionally freeze while near but not touching water, and will form obsidian or cobblestone.
 * When a player is inside a lava block, spiders, cave spiders and endermen appear red due to the eye textures being mostly blank. This also happens with enchanted items due the additional glint texture they have.
 * If redstone is placed next to water in such a way that the water does not run through it, and lava is placed directly over the redstone, where the redstone was previously will turn into obsidian and the lava source block will still be able to be picked up.

Trivia

 * A water source block placed 1 block away upwards diagonally (but not through corners) from a lava block will first flow in the direction of the lava, then other directions facing away from the lava. This happens because water physics treat the place that lava occupies as empty, and try to flow to it. Once the water turns the lava into obsidian, the water physics update to flow in all directions.
 * Above-ground lava lakes seem to generate most often in Taiga biomes.
 * An easy way to 'fill-in' large pools of lava is to use gravel or sand, which will fall to the bottom of the pool and stack upwards.
 * You can see the stars when in lava, even though it's supposed to be opaque.
 * If an item falls into the edge of a lava pool, it may pop back out and appear as if it were burning. Also, you can pick the item back up once it's back on land. Occurs more commonly in SMP.
 * Although lava is a liquid, it is not possible to drown in lava. This applies to all mobs.
 * Unlike water, lava does not stop a player's fall, though 9 continuous blocks of lava will mitigate any fall damage.
 * Arrows shot by the player will only catch fire if shot in flowing lava and not still lava.
 * You cannot put 4 blocks of lava in a 2x2 square or 1x3 line to create an infinite source of lava like you can with water, even in the Nether.
 * End Portals can be seen yellow if exposed to the surface when you are in lava.
 * Lava can also be used as a garbage disposal by dropping unwanted items into it.
 * Lava can set off tripwires, because they break placed string. It will only trigger it once.
 * While the player is in lava, all entities appear bright red.
 * Lava is the only block that is much easier to destroy with 2 TNT than with 1. The first TNT makes it start flowing, which allows the second TNT to destroy it.
 * When the player is in a bed, they cannot be damaged by lava.
 * When you are submerged in lava with the Night Vision status effect, the players view turns a brighter orange than without the effect.