Picture this: a subterranean hideaway so exclusive, it's been sealed off from the outside world for a cool 5.5 million years.

Yep, the Movile Cave is a strange alien universe right here on Earth, tucked underneath the Romanian coast. 

It may be just 20 metres below ground, but the Movile Cave is about as alien an environment as you will find anywhere on Earth. With no interference from the plebeian air that we breathe and light that we see, the cave instead is a concoction of sulphuric doom - the air low in oxygen, thick in carbon dioxide and sulphur to create a truly hostile environment. 

Except, bizarrely, life thrives there - an entire ecosystem having evolved in this toxic soup. Over 50 species have been discovered inside the cave, with 37 of them found nowhere else on Earth. And they are truly bizarre. Starved entirely of sunlight, Movile Cave operates on a dark magic known as chemosynthesis. In this underworld, foam mats of autotrophic bacteria serve as the base of a food pyramid that includes creatures like leeches, spiders, pseudoscorpions, woodlice, centipedes, water scorpions and also snails. Evolution in this harsh environment also means that the critters there can have no eyes and translucent skin (a sci-fi nightmare indeed). 

This Romanian oddity is off-limits to the public due to its extreme sensitivity. Only a select few researchers have been granted access to the enigmatic depths of Movile Cave, and scientists are determined to protect its unique ecosystem from any unwanted disruption. Fewer people have been to this cave than have been to outer space. Think about that. 

Movile Cave's ecosystem offers a sneak peek into life's resilience and inventive adaptability, and provides an excellent terrestrial analogue for deep-sea hydrothermal vents and extraterrestrial ecosystems, favouring the scenario of life existing subsurface beneath moons like Europa or Enceladus. 

So perhaps there are actually aliens out there - they are just right beneath our feet.