Ruins

"A relic of an ancient civilization long dead from the ravages of time."

- Wickerbottom

The Ruins are the last remnants of a forgotten civilization located on a level below Caves. They are generated as a new world upon entering them, just like caves. Ruins have their own theme, terrain, and biomes. Various unique creatures, plants, and other objects reside within.

Exploration
The ruins have six different Biomes; Abyss, Labyrinth, Military, Sacred, Village, and Wilds. The Ruins can only be entered through a plugged Sinkhole found in the Caves. Upon mining the Thulecite plug, a rope will lead down to the Ruins through the Sinkhole, and Batilisks will be able to spawn from it into the Caves.

The initial starting area of the Ruins is surrounded by four massive pillars and four Broken Clockworks, with a rope leading up to the Caves. The Ruins are completely dark, causing a steady drain on Sanity. As the Ruins are explored, the map will show new icons and terrain specific to the Ruins.

Earthquakes will occur in the Ruins, dropping the same minerals as in the Caves. Although time continues to pass on the surface world, there is no indication of this while underground.

Nightmare Cycle
The Ruins exist in a state of flux called the Nightmare Cycle, which can be measured by the Thulecite Medallion. As the magical state becomes more intense, the likelihood of earthquakes increases. Its current state will affect the state of objects within it. Nightmare Lights will begin to open up, the Runic Turf will glow red, and Ancient Statues will have a horrified look while Nightmare Fuel drips from their mouths and eyes. Shadow Creatures can attack the player regardless of their sanity level during the most intense stage. This is a great way to collect Nightmare Fuel, but is a bit overwhelming at an early stage in the game. The runes on the ground do not provide a light source despite glowing brightly.

Creatures
The following creatures can be found in the Ruins:
 * Depths Worm
 * Shadow Creatures
 * Slurper
 * Splumonkey
 * Dangling Depth Dweller
 * Damaged Bishop
 * Damaged Knight
 * Damaged Rook
 * Ancient Guardian

Friendly damaged Clockwork Monsters can be made "Followers" by repairing Damaged Clockworks with Gears.

Features
Some of the ground and pillars in the Ruins are made out of a special material that starts to glow when light is shined upon it during the Nightmare Phase. The Military and Sacred biomes are mostly covered in this material. Also somewhere in the ruins there's a labyrinth, filled with Dangling Depth Dwellers, Ornate Chests and dead ends. At the center of this labyrinth resides the Ancient Guardian, who most players will challenge to get its rare Horn.

Unlike Caves, surface plants like Evergreens, Saplings. Berry Bushes, Spiky Bushes, and Flowers are not found in Ruins, although the player can still plant these items. Plants will regenerate after a few days of being harvested (except for trees). The only plants that can be found naturally in the ruins are Light Flowers, Cave Banana Trees, Ferns. Lichen is also exclusive to the ruins. Blue Mushrooms can be found here as well.

Objects found in the Ruins include Relics, Ancient Statues, Nightmare Lights and the Ancient Pseudoscience Station. Relics can be attacked and destroyed for various loot while Ancient Statues can be mined to yield Thulecite and various Gems. Nightmare Lights can emit light and spawn Shadow Creatures depending on the Nightmare Cycle, while the Ancient Station will be used to craft some of the best items in the game, found in the Ancient Tab. Also in the wilds, there are Splumonkey Pods and special Ponds. The ponds can be fished in to get Eels.

Within the entrance of the Ruins, corpses of the Chess Monsters can be found. When destroyed, they yield 1 Frazzled Wires, as well as potentially Gears and other trinkets.

Reign of Giants
In Don't Starve: Reign of Giants, the temperature of the Ruins depends on the temperature of the overworld.

Trivia

 * The Ruins were introduced in The Stuff of Nightmares update.
 * Since the release of the Ruins had to be pushed back, the Slurper was made able to spawn in regular caves until the Stuff of Nightmares update, which made the Slurper a ruins-only creature, as intended.
 * According to Maxwell, the over-use of fuel from Nightmare Lights led to the downfall of the civilization that once lived in the Ruins.
 * Without mineral eating mobs like Rock Lobsters and Slurtles, the minerals generated by earthquakes will amass over time. This can cause lag, if there are too many around.