This mine was left deserted by Twolegs many many moons ago. There are many underground tunnels that are filled with mice and other small prey. However, there are also rats, so going down into the mine shafts sometimes is risky if cats are not in groups. Cats can get from place to place in the mine by riding in a tiny monster, which is actually a small cart fixed to an old rusty track. At first the cats were wary to get inside the 'monster' but over time they realized that it could effectively transport them to deeper parts of the mine without having to deal with rats.
