In a dark cave in Tabasco, southern Mexico, near the village of Tapijulapa, lives a community of molly fish. There would be nothing remarkable about this, except that the cave is poisoned with hydrogen sulphide, a toxic gas deadly to life. The fish have developed a suite of remarkable adaptations to their dark, toxic world. They may even have come up with a way to survive a local religious ritual that involves poisoning them. In fact, the cave mollies seem to be well on their way to becoming a distinct species. That might seem odd, given that there is no barrier separating them from the light-drenched pools outside the cave where regular mollies live. But in this case, the reasons might be sticking to the cave for a rather direct reason. Lurking at the cave mouth is a ferocious, gruesome predator.
A fascinating example of evolution in action and a setting with wonderful possibilities. A dark cave system filled with poisonous gas, strange and unique life plus dangerous predators, just ripe for a TARDIS to materialise.
Or it could be relocated to an alien planet, ready for more adventures.