You know when people start talking about how life can exist in ways we can't imagine, like methane-based life forms on Mars or Titan or whatever, and you're like 'woah cool' but deep down you're like 'yeah wishful thinking, whateva bruv', well let me add a nail to your subconscious coffin. Allow me to introduce intraterrestrial life; life that exists beneath you, beneath the ground, beneath the sea, down, down, into the earth's crust.

Intraterrestrial life

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Let's start at the bottom of the sea. Previously, I explored the sheer abundance of life that can be found in the deep ocean under extreme pressures, and continued with the deepest fish ever, the snailfish and concluding with barophilic extremophiles which just go beyond sense.

Naturally, I wanted to take it further but I had other things to write about. until now! And it's definitely worth looking more into - It turns out as much as 1/3rd of the Earth's entire biomass exists intraterrestrially. We should not ignore such huge contributors to life!

Most of this mass is found tens of metres beneath the ocean floor in mud. If you think about it, this kind of makes sense. Floating around 200 metres above the ocean floor is probably quite nutrient-poor. many of these organisms survive on marine snow, bits of dead stuff floating down from below, or a whale carcass if they're lucky.

Go a little further down, however, and all that snow has landed and congealed into a planet-wide buffet. Perfectly delicious, free and abundant. A lot of the oxygen rich sources that allow bacteria to survive down there stops a mere 1-2 metres below the mud, but in other places oxygen can be found 80 metres or more below.

This is a little exotic and unknown but one unusual idea is that radioactive decay could be the source of this oxygen (Radiolysis), splitting it away from the Hydrogen in water (H₂O). Exotic, but feasible. Bacteria that live on or under the bedrock making their own food out of organic chemical production are catchily called chemolithoautotrophic in nature. Remember that, I'll test you on it later.

The chemolithoautotropic bacteria don't just sit around stationary in mud, however. They actually like to travel around in volcanic superhighways; streams ocean current and nutrients in tiny cracks in the volcanic crust.

The bacteria are not alone, however. Whenever I find bacteria surviving in extreme locations, I also read about the existence of archaea, viruses, fungi and more. Now consider the amount of floor space they have to travel around on the ocean floor.

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33% of all life doesn't sound so unlikely now, does it?

Can we go even deeper?

I thought you'd never ask. I mean, 80 metres below the ocean is like... barely getting through the skin of an apple. That's pretty pathetic. I thought life was extreme!

So, a team decided to get drilling under the ocean. At first, they had to get through that muddy sediment layer and then through some tricky basalt rock and into what is called the gabbroic layer, which makes up the majority of the ocean crust. To make life easy, they went with an area called Atlantis Massif, because some volcanic activity conveniently pushed the gabbroic layer up to barely 70 metres beneath the ocean floor. But why stop there?

The team from Oregon State University decided to go a full 1,390 metres down, and though you'd think this is way, way away from the earth's core, the temperatures at this point reach a comfortable boiling point, or 102°C, likely due to volcanic activity.

Here, to no surprise by you guys at this point I'm sure, they found life! Much like the Aeroplankton I discussed before, at these depths, life was sparse, but definitiely there. Previously, life in those basaltic layers had already been found, so you'd think a similar form of life would be found a little further down, but that was apparently not the case.

Archaea apparently gave up at the basaltic layer and were thus absent from here. Another big difference was just how extremophilic these bacteria were. These were the kinds of bacteria you find in dirty gutter oil, thriving off... methane and benzene.

Those methane lifeforms on mars not sounding so far-fetched now, are they?

But can we go even deeper?

Getting adventurous are we?

well, according to this research, when digging into continental crust, life has been found 2,800 metres beneath sedimentary rock, and 5,300 metres beneath igneous rocks. Not only that, but the researchers of the earlier experiments explained that the stuff those oily methane extremophiles thrive on are found far deeper, in the earth's mantle itself, so, if pockets of coolness exist down there, why not?

And that is the limiting factor here; not depth, but heat. We know life can survive some pretty impressive temperatures like those ants I wrote about and various smaller things that can go even further, but there really is an absolute limit, and the deeper you go, the hotter things get. But given the distribution of muddy, sedimentary, gabbro-ey surfaces out there, many scientists suspect there may even be an equal amount of biomass in the earth as on it.

But the greater implication here is that if life can survive on methane beneath the earth's crust, thousands of metres up in our atmosphere, frozen for millenia in the Siberian tundra, in salty and toxic, boiling volcanic vents, in our acidic stomachs, in oil, in plastic, is life just inevitable?

I've written about marine spiders that literally live in the ocean, others that perpetually live in little air pockets underwater in rivers hunting tiny fish, other that live atop himalayan mountains eating organic life that happens to blow up to them in an updraft, ants that thrive in the hottest, most desolate places on earth. and I'm nowhere near finished with this series.

Well, how about that solar system, eh? Let's go take a look.

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Sources: _{Radiolysis | Colonization of subsurface microbial observatories deployed in young ocean crust | Under the sea: microbial life in volcanic oceanic crust | Abundance and diversity of microbial life in ocean crust. | First Investigation of the Microbiology of the Deepest Layer of Ocean Crust | Exploration of deep intraterrestrial microbial life: current perspectives}