Canyon

A canyon (commonly referred to as ravine) is a deep, long crack generating in the Overworld that can connect with caves. They often appear deep underground but can breach the surface.

Structure
Canyons are usually between 85 to 127 blocks in length, and typically less than 15 blocks wide. $$ canyons are usually 27 to 62 blocks in depth. Canyons $$ can sometimes go all the way down to bedrock, causing canyons spawning in mountain biomes to appear deeper.

A canyon extending down to bedrock causes lava to generate at the bottom. Obsidian often forms at the bottom as water sources flow down into the lava. Because canyons can reach below Y coordinate 15, it is possible to find diamond ore exposed near the bottom.

Canyons can have small ledges along the top. They can be found at levels 10 to 72, sometimes appearing on the world surface or underwater, forming canyons. They can go 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 10. Canyons can cut from surface level all the way down to bedrock, especially noticeable in the Amplified world type.

Canyons can also be found in oceans or rivers having a waterfall from the river/ocean falling into them. Water-filled canyons (1.13) can spawn in the ocean with magma blocks at the bottom forming bubble columns as well as obsidian between. 1.13 gives them unique underwater generation, of higher floating solid blocks in interesting patterns.

Canyons can connect to caves, underwater caves, dungeons, abandoned mineshafts, and any other generated structures. Due to the large surface area of their walls, canyons often have water and/or lava flowing down them (from springs in the walls, underground lakes, or openings to the sea). A canyon that intersects a river, frozen river, or swamp is filled with stone under the body of water.

Lava oceans


Similarly to how water oceans replace all air blocks below a certain altitude on the surface, when deep enough in a canyon, all blocks that would generate as air instead generate as lava.

Trivia

 * $$ the seed 1669320484 generates a huge canyon extending endlessly from southwest to northeast at coordinate X=−1269, Z=0, shaped by countless identical connected north-south canyons. There are other lines of canyons parallel to this one, but few of them are visible aboveground or have the canyons connected. This seed also generates parallel lines of endless dungeons underground.
 * It is relatively-common to find up to around half a dozen canyons intersecting close even to where they're first found (including vertically, often perpendicularly). So if one finds one, one may find more, soon-enough.