What extremophile organisms have been found deep inside the Earth?

The answer

The deep biosphere - life existing within Earth's crust - contains an estimated 10-30 billion tons of carbon in microbial biomass, roughly equivalent to all plant life on the surface. In 2019, the Deep Carbon Observatory reported discovering microbes 5 kilometers underground in South African gold mines, surviving at temperatures of 60°C in rocks 2 billion years old. These organisms, including Desulforudis audaxviator, derive energy from radioactive decay of uranium splitting water molecules - completely independent of sunlight. Some deep microbes reproduce only once every 10,000 years. In 2023, researchers found nematode worms at 1.3 kilometers depth in South African mines, the deepest multicellular animals ever recorded.