Lava

Lava is a fluid block that can only be placed by the player when using a bucket. It emits light and sets surrounding blocks 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 unsavory 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.

Burning

 * Most entities, including players and mobs, will take damage every half-second from being in contact with lava, in addition to being set on fire. If the entity touches water during this time or rain falls on it, the fire will be extinguished.
 * 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 are flammable; in 1.2.5 they are not (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. Also, if gravel falls vertically into lava, it will burn but only in the same way as entities do (the fire will not spread and the block isn't destroyed). Ice melts into water, TNT explodes.
 * The “embers” or “fireballs” which fly out of lava are purely decorative and do not directly cause fires or cause damage to entities.
 * In Beta 1.6.6, lava does not ignite anything below it, just above it and to its sides. This can create interesting structures.
 * 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'll 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. Contrary to popular belief, lava cannot remain forever with out a source block. It may take several hours, but it will always disappear. 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. It travels at a rate of 1.5 blocks per second.

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

 * Since 1.9 Prerelease 1, 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.
 * In minecraft version 1.3.1, there is a strange possible glitch or new feature causing lava and water with cobble separating them to cause damage to a player who is standing in the water touching the cobble, almost like the water is hot and burning the player. (This may be a flow bug: which may have been later-fixed.)
 * 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


The primary natural source of obsidian is rivers flowing onto the surface of lava pools in deep caves; care is needed mining these since there is usually a layer of lava under the obsidian.

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 glowstone, TNT, leaves, and other "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 can be placed in a furnace in a bucket to be substituted for coal. Its burning time is 1000 seconds, compared to 80 seconds for coal (lava smelts 100 blocks, coal smelts 8). Since 1.3, you retain the empty bucket.

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.
 * Lava can still be blown up by TNT, assuming that the TNT block detonated in or next to the source block. This appears to be random.
 * When a player is inside a lava block, spiders, cave spiders and endermen appear red.
 * 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.
 * Sometimes Lava will not float if placed on a sign in Multiplayer.
 * Sometimes, Lava will float, and updating it will not stop it from floating.
 * All the dripping particles for lava are black in the Xbox version.

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.
 * Lava can be contained by surrounding it with snow blocks, because they are not flammable and do not melt.
 * It is a common misconception that underground lava pools cause more minerals to spawn. The idea arrives from the fact that the large open area leaves more of the nearby natural minerals exposed. If anything, the air pockets associated with lava pools actually reduce the chances of minerals spawning by taking away natural rock space.
 * 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 if 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.
 * Arrows shot by the player will only catch fire if shot in flowing lava and not still lava.
 * Lava CAN be used for fuel in the furnace and can smelt a little more than 1.5 stacks of ore/material. An empty bucket will remain in the furnace, ready to reuse. (1.5 stacks = 96; actual smelt is 100.)
 * 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.