Lava

"Today, lava spawns naturally in Minecraft in a whole bunch of places. The easiest location to find it is in the nether, where it forms vast seas below y-level 31. But it'll also spawn in the overworld, replacing air blocks in caves at the bottom of the world. Be very careful digging through rock below y-level 10 because it's very easy to find yourself face-to-face with a surprise lava pool that incinerates all the precious diamonds you just painstakingly collected. No point crying about it. The lava will just incinerate those tears too."

- Duncan Geere

Lava is a light-emitting fluid block that causes fire damage, which generates below y-level 10 in the Overworld and below y-level 31 in the Nether.

Natural generation
During world generation, lava replaces air blocks generated in caves and ravines between levels 1 and 10. Lava does not replace air blocks inside mineshafts, dungeons or strongholds between y-levels 1 and 10.

Lava can also occur as lava rivers from a single spring block, pouring down walls into pools. The spring block can be on the side of a cave, ravine, mineshaft, or stone cliff above ground.

Lava also spawns as lakes, which can be found at any elevation within any biome.

Two blocks of lava can also be found in plains, snowy tundra, desert and one of savanna villages weaponsmith houses. 15 blocks of lava can be found in the End portal room of a stronghold: 3 along the left wall, 3 along the right wall, and 9 below the portal frame. Lava also generates in woodland mansions, 2 block of lava generate in "blacksmith room" and 25 block of lava generate in secret "lava room".

In the Nether, lava is extremely common, appearing more frequently than water in the Overworld. Seas of lava occur, with sea level at y-level 31, about a quarter of the total height of the Nether (as the Nether is 128 blocks tall). They extend down to about y-level 22 at the most. Lava also randomly appears in single blocks inside netherrack formations. Lava also generates in fortresses in wells in the Nether.

Obtaining
Lava cannot be obtained as an item, but can be retrieved with a bucket. $$, it may be obtained as an item via inventory editing.

Burning


Most entities take damage every half-second while in contact with lava, and are set on fire. When in contact with lava, 300 Fireticks are added instantly to the Firetimer of the entity/player. For every further tick the player is in contact with lava, 2 Fireticks are added to the Timer. For example: 10 seconds in lava cause a total amount of 700 Fireticks (35 seconds) where the player burns (300 initially + 400 Fireticks for 200 ticks being in the lava) or rather 500 Fireticks (25 seconds) left to burn when they leave the lava source. If the victim touches water or rain falls on it, the fire is extinguished, but the lava continues to damage them directly.

Nether mobs (which are immune to fire), vexes, or players or mobs affected by a potion of fire resistance are not damaged when touching lava.

The embers or fireballs that fly out of lava are purely decorative and do not cause fires or damage to entities. When rain falls on lava, the black ember particles appear more frequently.

A player in lava lasts a few seconds before dying, assuming the hunger bar is always full:
 * 2.5 seconds with no armor
 * 3.5 seconds with full leather armor, no enchantments
 * 5 seconds with full gold armor, no enchantments
 * 5.5 seconds with full chain armor, no enchantments
 * 6.5 seconds with full iron armor, no enchantments
 * 14 seconds with full diamond armor, no enchantments

Fire spread
Lava can cause fires by turning air blocks to fire blocks:
 * in a 3×3 area right above the lava, and
 * in a 5×5 area above that.

In order for air above lava to turn to fire, a block adjacent to the air has to be flammable, or one of the wood-constructed non-flammable blocks. Since catching fire depends on air blocks, even torches or lava itself can prevent a flammable block from catching fire. Additionally, not all flammable or wood-constructed blocks can be ignited by lava.

Lava of any depth can start fires this way, whether or not it appears to have a current.

Flow


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.

In the Overworld, lava travels 3 blocks in any horizontal direction from a source block. Lava flows far more slowly than water (1 block every 30 game ticks, or 1.5 seconds), and sourceless lava flows linger for a short time more. In the Nether, lava travels 7 blocks horizontally and spreads 1 block every 5 game ticks, or 4 blocks per second, which is the same speed as water in the Overworld.

Flowing lava destroys 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, repeaters, end rods, and rails. Sugar canes hold back lava, but disappear if the sugar cane's water source is destroyed by the lava.

Using redstone wire, a one-block lava flow can be redirected by supplying power to the source block, which causes it to reset the flow towards the now-nearest terrain depression. 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.

Flowing lava does not push entities, unlike flowing water.

Lava and water
Water and lava can produce stone, cobblestone, or obsidian based on how they interact.

Light source
Lava blocks emit a light level of 15.

Other


Lava above a non-transparent block (does not include stairs, fences, and slabs) produces dripping particles on the underside of that block. These droplets are purely aesthetic, functioning identically to their water counterparts, except that they drip slower.

Flowing lava can set off tripwires, because it breaks placed string. Lava triggers a tripwire only once.

An arrow shot into flowing lava catches fire, but not if shot into still lava.

ID
Java Edition:

Bedrock Edition:

Lava spends most of its time as stationary, rather than 'flowing' – regardless of its level, or whether it contains a current downwards or to the side. When specifically triggered by a block update, lava changes to 'flowing', update its level, then change back to stationary. Lava springs are generated as flowing, and lava lakes are generated as stationary.

Block data
If bit 0x8 is set, this liquid is "falling" and spreads only downward. At this level, the lower bits are essentially ignored, since this block is then at its highest fluid level.

The lower three bits are the fluid block's level. 0x0 is the highest fluid level (not necessarily filling the block - this depends on the neighboring fluid blocks above each upper corner of the block). Data values increase as the fluid level of the block drops: 0x1 is next highest, 0x2 lower, on through 0x7, the lowest fluid level. Along a line on a flat plane, lava drops one level per meter in the nether and two everywhere else.

Trivia

 * The player can swim in lava without taking damage if they are under the effects of Fire Resistance, although the swimming speed is noticeably slower. Additionally, flowing lava doesn't move players/mobs. A player cannot sprint-swim on lava.
 * A water source block placed 1 block away upwards diagonally (but not through corners) from a lava block first flows in the direction of the lava, then other directions facing away from the lava. This happens because water physics treat the lava block 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. (The same thing happens with lava flowing over water.)
 * Although lava is a liquid, it is not possible to drown in lava. This applies to all mobs. However, it is still possible to suffocate in lava. This applies to almost every mob.
 * A player sleeping in a bed cannot be damaged by lava.
 * If lava is changed to be transparent via a resource pack, it does not become transparent.
 * Lava does not absorb fall damage unlike water.