Mars is a pretty astounding planet, and our missions to Mars have been making fascinating and ground-breaking discoveries for decades now.

But some of the coolest Mars research isn’t actually conducted on Mars. It’s done here on Earth, in environments that are a lot like Mars, either as it is now, or as it was billions of years ago.

They’re called terrestrial analogues. And the research done in these environments has changed the way we think about life on Earth, Mars, and rocky planets in general. There are a couple main reasons to study terrestrial analogues for Mars.

One is that it’s a practical approach to space research. It’s difficult and expensive to get to Mars, and we’re already here on Earth for free. And we have way too many questions about Mars to be able to answer all of them with just the tools we have over there.

So doing Mars-related research on Earth lets us learn more about both Mars and Earth than we would if we only did our Mars research on Mars. Another reason is that the best way to solve some Martian mysteries is to compare Mars to Earth. One of the biggest questions when it comes to Mars is whether it ever harbored life.

And looking for life in places on Earth that resemble Mars can give us a better idea of what kinds of adaptations life might have developed to survive on Mars, if it ever did evolve there. Knowing more about where life can theoretically survive could also help us figure out where to look for signs of life on Mars. So, some of the best analogues for Mars here on Earth are useful not just because of the insight they give us into Mars as a planet, but because of the insight they give us into Mars as a potentially habitable planet.

Like the Naica mines in Mexico, for instance. The Naica mines and caves are probably similar to underground environments on Mars, which we know exist, but haven’t been able to explore because it’s super dangerous to send a rover underground on another planet. The caves at Naica are probably especially similar to what it would have looked like underground on early Mars, when the planet was much wetter and warmer.

Like most mines, the Naica mines are deep underground, but unlike most mines, they’re ridiculously hot and humid. Like, lethally hot and humid. Researchers have to take tons of precautions, including wearing special “ice suits” with oxygen supplies, to make sure they don’t die.

The mines also happen to be incredibly beautiful, home to huge caverns containing massive gypsum crystals that dwarf elephants, let alone people. And from experiments started around 2009, researchers discovered something incredible: there were dormant microbes in fluid inclusions in the crystals, basically tiny little pockets of water that form in a crystal as it grows. And the researchers were able to revive them!

That tells us two things: first, that if life ever evolved on Mars, it might have been able to survive in similar cave environments; and second, that those are really good places to check for signs of life, past or present. This strategy of surviving in rock is really weird, but super useful. And a similar strategy has been taken up by the microbes living in another place on Earth that’s a great analogue for Mars: the McMurdo Dry Valleys in Antarctica.

The Dry Valleys are basically the opposite of the Naica mines: they’re super cold deserts, and they’re a lot like the dry, freezing lowlands of the Martian north pole. Researchers working on projects for places like NASA use the Dry Valleys as a place to test equipment destined for Mars, and astrobiologists use them to explore Mars’s potential for habitability. Because even though the Dry Valleys are really cold and dry, scientists have discovered a few forms of life that manage to live there. And some of them have adopted a similar strategy to the life in Naica, despite the huge difference between their habitats. There are endolithic phototrophs in some of the rocks at the Dry Valleys. Endolithic means “inside rock,” and phototrophs use photosynthesis. And that’s what these organisms do: they live inside rock, but they still use photosynthesis. The rocks containing the endoliths are mostly sandstone, which can transmit some light through it. So the microbes inside the rock are still able to photosynthesize even though they’re not directly exposed to sunlight, and they get a nice little rocky home to protect them from the harsh Antarctic desert.

Both Naica and the Dry Valleys host life that has taken an approach to survival that could be outstanding on Mars. Since Mars doesn’t have much of an atmosphere and has no magnetic field, its surface is constantly bombarded by UV light. If potential life on Mars lived inside rock or underground, that might be enough shielding from radiation for them to have survived for a good while during Mars’s early history. And the neat thing about these strategies, especially the endolithic strategy, is that it can work anywhere you have the right kind of rock. This could work just as well at Mars’s north pole as it could in its southern highlands, as long as the rock is transparent enough.

So, these discoveries have given us a window into Mars, and we didn’t even have to leave Earth! As we continue to explore beyond our solar system and find rocky exoplanets, this research becomes even more important. It helps us define what it means to be habitable for all planets, not just our own. And a bunch of little underground microbes just gave me an existential crisis.

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