On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvental ecosystems, supporting large complex life forms such as "Riftia pachyptila". Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone. Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the "Milnesium tardigradum", which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. In addition, the plants "Rhizocarpon geographicum" and "Xanthoria elegans" have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. The fungi "Cryomyces antarcticus" and "Cryomyces minteri" are also able to survive and reproduce in Mars-like conditions.