Overall characteristics | Ruins Don't Starve Guide

The Ruins are in the second level of the caves and are automatically generated. Beware of earthquakes. Their frequency grows with the increase in magical flux (it's variable). High magical flux causes Nightmare Light to produce light, and the red veins on the ground to glow brightly. When this happens, Ancient Statues are slowly being covered in dark matter. You will experience hallucinations regardless of your sanity level - so get ready for a fight. The entrance to the ruins is in the caves. First, though, you have to clear the rubble that covers the entrance.

To go deeper you have to use a pickaxe.

There are three types of terrain in the ruins. On sand-coloured terrains you will find Fern, Lichen, Barrel, Wild Pond, and Light Flower. On the darker area there are spider lairs, Nightmare Lights, and Ornate Chests - the last can be crushed with a hammer. The third type of terrain contains Ancient Structures and Nightmare Lights.

Ruins are really vast.