Paris Catacombs, France. Since the 13th century, the ground beneath Paris has been mined for limestone and other minerals, so the entire ground that the city sits on is a labyrinth of tunnels. A little over half of those tunnels are mapped, while the rest are left to speculation. Back in 1800’s, the graveyards of Paris were getting so full, that they had to start transferring the bones to the tunnels, and arranged them in a Gothic display. This ossuary (where the bones are kept) are open to the public for you to check out and even touch, if you wanted to. If that’s your thing. We won’t judge.