Also longish but interesting. Have not read much
about subsurface organisms like these.

https://www.bbc.com/future/article/20240821-could-alien-life-survive-in-deep-lakes-below-mars-surface

To understand the life that might survive deep
below Mars' surface, we can look to some of the
deepest, and oldest, forms of living organism
on our own planet.

Mars isn't just the red planet: it's also a wet
planet. On 12 August, US researchers reported
evidence of a vast reservoir of liquid water,
deep in the rocky crust of the planet.
...
Underground water on Mars opens up the
possibility of underground life on Mars. The
last few decades have revealed that there is an
enormous biosphere hidden deep within the
Earth. It now seems the same could be true on
Mars. Martian life, if it exists, could well be
subterranean.

For over 30 years, biologists have accumulated
evidence that life persists deep underground on
Earth. Researchers have drilled deep into the
sea floor and the continents, finding life in
buried sediments and even amongst layers and
crystals of solid rock.

Most of these dwellers in the dark are
single-celled microorganisms, specifically
bacteria and archaea. ...

Within the last 20 years, it has also emerged
that the deep biosphere is highly diverse.
"There's actually quite a lot of different
types of organisms living deep underground,"
says Cara Magnabosco, a geobiologist at ETH
Zurich in Switzerland.
...
Because it is pitch-black, these microbes
cannot get energy directly from sunlight, as
photosynthetic organisms at the surface do.
"The really important thing to note is that
they don't depend, by and large, on the Sun,"
says Lloyd.

They also aren't receiving any other inputs
such as nutrients from above. Many of these
deep ecosystems are "completely disconnected
from the surface", says Magnabosco.

Instead, these ecosystems are based on
chemosynthesis. The microbes get their energy
by performing chemical reactions, taking in
chemicals from the surrounding rocks and water.
For instance, they may use gases such as methane
or hydrogen sulphide as their source material.
...
Furthermore, there are many other obstacles to
life in the Martian subsurface. "Life doesn't
just need water," says Lloyd. "It needs energy
and a place to be, so it needs a habitat." We
don't yet know if the pores in the Martian rock
are large enough for microbes. Likewise, the
chemical makeup of the deep rocks is crucial, as
they would be the source of chemical energy.

For Magnabosco, "the biggest uncertainty" about
life on Mars "is whether or not it emerged".
Because we don't know how the first living things
formed from inanimate material, we don't know if
conditions on Mars were ever suitable for the
emergence of life.
...


Had to look this up:
https://en.wikipedia.org/wiki/Deep_biosphere

"The deep biosphere is the part of the biosphere
that resides below the first few meters of the
surface. It extends down at least 5 kilometers
below the continental surface and 10.5 kilometers
below the sea surface..."


https://www.mdpi.com/2076-3263/10/11/461
Tracking the Deep Biosphere through Time
6 November 2020

"The deep biosphere is the second largest
reservoir of live biomass today, only surpassed
by land plants."

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