Movile Cave
This cave in Romania strikes most people who enter it as bizarre. Caves give most people the heebie geebies, but this one is a bit different... Its intense sulfuric smell sends people running. The cave hasn't seen sunlight in over 5.5 million years, which means a lot can brew beneath the surface...
There is a lake filled with sulphuric water sitting at the bottom. The air inside of the cave contains hydrogen sulfide which carried 100 times the amount of carbon dioxide than the surface. There are 33 unique species of animals and insects that live there. Most people who enter are pretty keen on leaving ASAP.