Cave



Caves (also known as caverns) are commonly-found underground structures generated in the Overworld and, to a lesser extent, the Nether.

Generation
Caves are randomly generated patches of air blocks (lava source blocks at layer 10 and below) underground, leaving exposed stone and other blocks generated with the terrain (such as mineral veins). They consist of a series of irregular tunnels branching off and winding in other directions, which may connect to the surface of the map creating natural entrances to the cave. In jungle biomes, vines generate in caves near the surface. Sand often will fall into caves generated near the surface of a desert or beach; craters in the sand will alert the player to caves below the surface.

Caves generate at any altitude up to Y-level 128. Caves can be quite deep, extending from the surface all the way to bedrock. They frequently intersect natural structures such as other caves, dungeons, ravines, and abandoned mineshafts. Because of low light levels, hostile mobs and bats often spawn in caves deep underground.

In the upcoming 1.13 update, caves and ravines are able to generate underwater.

Structure
A cave can be classified into the following kinds of cave systems.

Hollow


Hollows are highly cave-like structures that generate in biomes such as Extreme Hills. They are not really cave systems in a broad sense, although they may sometimes intersect with one. They are essentially overhangs that have been completely closed off and can be seen as the inverse of floating islands. These are extremely rare on default worlds without customized world option and usually have floors corresponding to the biome (most commonly grass).

Small cave


Small caves are primarily located close to sea level and are unlikely to be linked to other caves. Because of their relative shallowness, they seldom contain anything rarer than small deposits of basic ores such as coal and iron. Such caves may have multiple entrances which are naturally lit by sunlight or moonlight. These type of caves almost always feature pools of water/lava within, likely caused by the generation of the water itself.

Medium cave


Medium-sized cave systems are commonly found within reach of other, often smaller caves, which are accessible via mining. These caves frequently contain large deposits of basic ores alongside small repositories of rarer materials - including lapis lazuli and gold. Such caves also regularly contain, at the very least, minor water or lava flows as well as the presence of dungeons.

Large cave


These are large subterranean systems, found deep within the earth, often close to bedrock. They hold large deposits of essentially every known type of ore - with rarer materials, such as diamonds and emeralds. Note that emerald ore is exclusively generated in Extreme Hills biomes. Such systems contain a number of lakes of all types, as well as the presence of major lava and water flows.

Circular cave


During the terrain generation process, large symmetrical and cylindrical caves of various sizes can infrequently be created underground which often merge with other cave systems. These unusual type of caves vary from roughly 5 to 22 blocks in height, and from roughly 11 to over 30 blocks in width and/or length, though larger examples are quite rare. Although not particularly of interest, from a mining perspective, they may appear unnatural and irregular in comparison to the surrounding terrain. While the smallest rarely contain many ores, the largest can be quite useful for mining as their sheer surface area rivals that of a large ravine.

Very rarely, dungeons and even strongholds can be found in these rooms.

Ravine


Ravines are tall, long cracks of air, usually measuring around 30 to 50 blocks in height, 84 to 112 blocks long and no more than 7 blocks wide. Ravines can have small ledges along the top. They can be found at levels 20 to 68, sometimes appearing on the world surface or underwater, forming canyons. They can go very deep underground, sometimes spawning slimes or exposing diamond ore. If they reach deep enough, they may also be floored by the lava lakes at level 11. In the Amplified world type, ravines can cut from surface level all the way down to bedrock. Ravines can also be found in oceans or rivers having a waterfall from the river/ocean falling into them.

Ravines can connect to caves, dungeons, abandoned mineshafts, and any other generated structures. Due to the large surface area of their walls, ravines often have water and/or lava flowing down them (from springs in the walls, underground lakes, or even openings to the sea).

Giant entrance


These caves have a large hole for an entrance and then spiral down, often cutting into coal and iron deposits. Rarely, these entrances can go nearly straight down, exposing rarer minerals.

Sea-access cave


These caves are often connected with the sea, and can be accessed from the sea. These act like underwater entrances to caves, often large ravines. When lit by lava, they allow the Player to see them from the surface of the ocean.

Since caves usually generate specifically to avoid water, true sea-access caves are rare and most are formed by gravity blocks collapsing inside.

Water lakes


Found in every part of the world, underground water lakes are the most common type of lake. Water lakes may occur anywhere from a mere 1-20 blocks below the surface, all the way to bedrock.

Lava lakes


These type of lakes are most often found in large caves deep underground, although they may rarely appear on the surface. Though lava lakes can be extremely dangerous, if you have a bucket of water on hand, dousing lava lakes can be a quick and easy method of obtaining obsidian.

Trivia

 * Caves will often be overwritten by sand and sandstone, as well as other generated structures.
 * Though ores are abundant in caves and often require little digging to get to, ore veins visible in the walls of the cave often contain little ore, as the cave cut through them when it was generated.
 * Although caves generate in the Nether, they are generally not as of much interest as Overworld caves, since the Nether itself is essentially just a huge cave. Overworld caves give free tickets through the terrain exposing many ores, while the only ore of interest in the Nether can be found everywhere already. However, a secondary lava "sea" is not generated in the Nether as with Overworld caves, and as such Nether caves can reach exposed bedrock.