The Ruins mod spawns systematically generated structures in your Minecraft worlds, similar to Villages or Strongholds. Every structure is defined by a template and configured to spawn under specific conditions. In a majority of these Ruins there are traps, treasure, and fights all to add to the excitement of the Hexxit modpack.
They spawn all over the world and in many different biomes, including at the bottom of the ocean.
If valuable ore seems too convenient in a random building, it probably is. This mod adds traps.
Structures also spawn in the Dimensional Doors dimensions, and can be seen in the map. Be wary not to fall in the void if you choose to pillar your way to them.