In the very deepest, hottest and inhospitable parts of some Gold Mines, there live some tiny worms which creatures are the deepest living animals that have ever been discovered.
How they got there is a mystery, as is how long they have been living there. There is not much food or oxygen down there. These creatures are a type of Nematode worm.
Tests have been done on the worms to make sure they are not being brought in from outside, and also it has been found that they prefer to feed on the bacteria present in the mine rather than those present on surface. Bacteria have been shown to live up to 12Km underground so who knows how far down the worms can go.
Nematodes are widespread through most environments in the Earth, from hot deserts to the top of cold mountains and also in the bodies of other livng beings, including humans.
They can survive oxygen levels down to just 0.5% and have been found to tolerate temperatures up to 61 deg C.
If things get really really tough they have another survival technique ready to deploy. They can go into a kind of stasis triggered by releasing a pheromone and slow their metabolism right down in what is known as the Dauer stage. Dauer is a non aging duration that does not affect the adult lifespan. In this stage they have survived re-entry into the Earth's atmosphere from space.
Other amazing survivors are a group of tiny animals known as Tardigrades.These are tiny (0.5-1.5mm) water dwelling, eight legged, segmented micro animals. They are some of the toughest of known organisms.
They can cope with conditions that would be rapidly fatal to nearly all other known life forms.
They can withstand temperatures from just above absolute zero to above the boiling point of water (100 °C), pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space.
Some were stuck on a space craft and not only survived the jouney but a few of the females had laid eggs.
They can go without food or water for more than 10 years, drying out to the point where they contain 3% or less water, only to rehydrate, forage, and reproduce once given the chance.
Most eat mosses lichens and algae, but some are carnivorous.
I think tardigrades would make the perfect pets.