Caves

Caves are large underground sections of the world filled with unique features. They are so large that they are generated as whole new maps upon entering. Caves have their own artwork, terrain, and maps. Various creatures, plants, and other objects reside within.

Spelunking
A Cave can be found underneath a Plugged Sinkhole. Upon mining the plug, an open Sinkhole appears with a rope leading down into a new Cave. Each Cave is generated just like a new world. A different loading screen (pictured above) will play while it is created. While within a Cave, time in the world above continues to pass and vice versa. Once a Sinkhole is unplugged, Batilisks (see below) can emerge at Dusk and roam the surface world.

Each Cave begins in a small circle of light with a rope hanging down and a skeleton at the bottom. The world within a Cave is always considered to be Night. This means that Sanity slowly drains, and in areas of complete darkness, characters are vulnerable to Charlie. Some natural light does leak in from the surface at places, however, and there are other light sources as well. When the map is opened, players will see a new set of icons and terrain specific to the underground (pictured at right).

Earthquakes
While underground, the player may experience Earthquakes. When this happens, the screen shakes and a rumbling sound plays. Rocks, Flint, Nitre, Marble, Gold Nuggets, and sometimes Red and Blue Gems will fall from the Cave ceiling. Each of these is preceded by a small shadow and causes damage if it hits the character. Many of the items that fall during an earthquake shatter when they hit the ground, but some will remain intact. Once the earthquake ends, Slurtles and Snurtles leave their homes and roam the Cave in search of rocks to eat.

Creatures
In addition to normal Spiders, Fireflies, and Tentacles, the following creatures reside only in Caves:
 * Cave Spider
 * Spitter
 * Batilisk
 * If there is an open sinkhole on the surface, Batilisks will spawn there at dusk and return in the morning.
 * Bunnyman / Beardlord
 * Bunnymen can be found above the ground only if the player builds Rabbit Hutches, which will spawn one Bunnyman per Rabbit Hutch.
 * Slurtle
 * Snurtle
 * Rock Lobster
 * Big Tentacle
 * Depths Worm

More creatures can only be found in the Ruins, below the Caves.

Features
Some surface plants are also found in caves, such as Saplings, Evergreens, Berry Bushes, Grass, and Mushrooms. Additionally, several plants only grow underground: Light Flower, Cave Banana Tree, Fern, and Mushtree and its Red and Green counterparts. Like the aboveground versions, Berry Bushes and Grass will regenerate after a few days.

Stalagmites are a common sight in any cave, and can be mined for Rocks, Gold, and Flint. Similar looking Spilagmites are home to Cave Spiders and Spitters, which can only be destroyed with a Pickaxe. Less common are the Slurtle Mounds, which spawn slow-moving Slurtles and rarely, Snurtles. Additionally stalactites and various rock formations can be found in the dark areas of the caves.

Nightmare Fissures can be encountered in the caves, which look like jagged cracks in the ground, with a color that changes based on the Ruins' place in the Nightmare Cycle. Like the Plugged Sinkhole to the Ruins, 2-3 Nightmare Creatures which can detect and attack the player regardless of sanity level will spawn nearby. If the player has experience with combat, and prepares/chooses his ground carefully, these monsters can be farmed for Nightmare fuel, though the constant sanity drain in the caves should ensure hostile encounters with nightmare creatures anyway.

Like on the surface, Set Pieces appear in caves too, often with a skeleton, and even with advanced buildings such as Ice Boxes, Science Machines, and Chests.

Death in Caves
Death in a cave is permanent, unless the player has a Life Giving Amulet, Touch Stone  or Meat Effigy, in which case it'll result in the player's health, hunger and sanity being reduced and any items carried being left where the player died, as with aboveground death.

Trivia

 * Caves were added in the May 21st Underground update.
 * If Hounds are supposed to attack while the player is in the caves, it will be skipped (rather than being "made up" later like sleeping for the night does). A new monster, the Depths Worm, has attack periods like Hounds. This makes the chance of Hounds underground unlikely.

Bugs

 * After sleeping in a cave, the morning tune and evening tune play at the same time.
 * If the player starts a fire above the ground and then goes underground, no matter how much time is spent in the cave, the same fire will still be burning upon the player's return.
 * When coming out of the cave when light is coming in it (in the evening it's best shown), light will persist in the above-ground area.
 * Sometimes if it passes too long since you visit a cave that you previously entered the game will generate a new cave.