Forgotten ruins may be found scattered throughout the underground to add some danger and interest to mining and extra exploration opportunities. These ruins may be complex structures that contain monsters and common/dungeon loot. Some can be found throughout all biomes, but there are also biome-specific ruin variants.
Underground ruin maps can sometimes be found in dungeon loot (for Overworld ruins) and Nether Fortress chests (for Nether ruins).
Sometimes there are markers on the surface above a ruin. If you see mossy cobblestone or cracked stone bricks that look out of place, dig down as there may be a ruin beneath!
Ruins often have monster spawners so may be located by listening out for many similar monster sounds while exploring underground. Additionally, ruin complexes may be quite large and intersect a cave or gorge.
Nether ruins spawn beneath the lava ocean so may be tricky to find, but they are also often large and sprawling. Nether Fortress chests sometimes contain maps that can lead you to them.
With permissions enabled, you may search for the closest underground ruin by using the command