There are plenty of places on Earth that seem alien to us. The deep sea is a perfect example: it's been said that we know more about Mars than we do about the bottom of the ocean.
Hydrothermal vent communities weren't discovered until 1977, well after the first landings on Mars. Scientists believed that the deep ocean was cold, dark, and inhospitable to life. But when scientists started studying the areas where hot mineral-heavy water wells up like an underwater geyer in tectonically active-regions (seafloor rifts), they were surprised to discover that deep-sea vents were home to thriving communities, full of life.
But they are dark, if not cold. If these deep sea communities had to subsist on detritus falling from the sunlit ocean surface, they would be more like cave organisms, rare, slow and sluggish in a low-energy environment. They weren't: there were lots of animals, a high biomass, and many were active. Without photosynthesis, where does the energy come from?
The main animals featuring in the above photo are giant rift worms. They grow to be up to 1.5m long, the fastest-growing known marine invertebrate. They have a bright red top part and a whitish shell into which they can withdraw. They were originally put into their own phylum, the Pognophora (one of my favorite animal names ever), but sadly they were reclassified as part of the Siboglinidae, a larger group of worms. (I'm going to keep calling these giant rift worms Pogonophorans, if only because I know how to pronounce it.)
As if it weren't strange enough to be living deep in the oceanic darkness, here's the weird part: these critters have no mouth, no gut, and no anus. And what's the advantage of being red in absolute darkness?
Giant rift worms are packed full of bacteria. Instead of relying on the energy from sunlight as photosynthetic organisms do, these chemosynthetic bacteria use hydrogen sulfide as an energy source for producing carbohydrates. The giant rift worms live off the products of these bacteria, in essence farming their own food inside themselves.
The red color is from hemoglobin, used to absorb hydrogen sulfide from the water and transport it to the bacteria. In mammals, hemoglobin is used to transport oxygen. Hydrogen sulfide is poisonous to us exactly because it bonds to the hemoglobin, blocking the oxygen. The pogonophorans have taken advantage of this transport capability, and have evolved a form of hemoglobin that successfully transports both hydrogen sulfide and oxygen.
Not all of the chemosynthetic bacteria are symbiotic. Free-living mats of bacteria form around the vents. All sorts of creatures live there, dining on bacteria: crabs, snails, fish, shrimp, and more. But most of them don't dine as directly as the pogonophorans.
Although they don't rely on the sun for energy, these communities do depend on sunlight in a more subtle way. Most of the organisms here get their oxygen from the surrounding water, and that oxygen comes from photosynthesis at the ocean surface. Still, these deep sea rifts offer science fiction writers a model of an ecosystem that could thrive on a moon of Saturn, or a planet with no life at the surface. The heat and chemical energy both come from geological processes. Pogonophorans also give us a model organism that doesn't need to eat or excrete.