A 'subterranean Galapagos' inside the Earth (2018)
vice.com
vice.com
Even those basic requirements are not as clear-cut as you'd expect. Ancient microbes can wake up after being exhumed from glaciers. Closed cave ecosystems that have been self-sustaining with minimal input for thousands of years. Goddamn tardigrades in the vacuum and radiation of space.
Even the very air is absolutely inundated with life. Mostly with fungal spores, but we all choose to not think about that.
Probably the only places we won't find life is where it's too hot for organic molecules to exist. Even then, I wouldn't be that surprised if we find some type of non-carbon self-replicating molecules in magma or something.
I guess that's the beauty of evolution. If an opportunity exists, there will (eventually) be a creature evolved specifically to take advantage of it.
My secret dream for extraterrestrial life is something living in the gas giants. Ben Bova's stories about vast Jovian whales really struck a chord with me.
Anybody who's tried making sourdough in two very far away homes is probably acutely aware of this haha
The prevalence of life everywhere on Earth, and the complete lack of life everywhere not-on-Earth in every place we've looked (so far) - the moon, Mars, asteroids, etc, etc - are equally fascinating.
Over 30 years ago Thomas Gold published "The Deep Hot Biosphere". A review in PNAS is here [1].
First, to nitpick: we know that most oil is from plants (marine phytoplankton especially), not dinosaurs.
Now the real point: in the 50s soviet scientists had the idea that abiogenic oil could exist, then found some. The fact that all oil does not come from biological life is accepted by the scientific community since at least then. It's a small fraction of proven oil reserves anyway, and take around the same geological time to get formed anyway, so this is more of a curiosity than useful knowledge.
But maybe it had time to generate some oil (or whatever kind of longer-chain hydrocarbons) when it was still active.
Oil won't be very useful on Mars as an ubiquitous fuel (there's little oxygen, mostly needed for artificial biomes and the occasional rocket), but certainly would be useful to produce plastics.
Thomas Gold's theory is that oil is created in the core and seeps up through the rocks, and it collects biomarkers along the way because stuff lives down there.
The mainstream theory is that oil contains biomarkers because it is made out of dead stuff, and Thomas Gold's theory that stuff lives down there can't be true because nothing lives down there (yes, circularly).
(Disclaimer: I have not read Thomas Gold's book, but I have a very enthusiastic friend who has. I might be wrong about what he says).
Mammals just require too much energy to function.
Beneath the surface of water there can be more surprises, especially around large sources of (volcanic) heat and substances on the sea floor.
Bright orange bats were discovered in West African mining tunnels in 2021. Even very big animals can be still discovered near home. A 12m long, new species of whale hiding a plain sight in the Gulf Of Mexico, and so critically endangered as the Vaquita, was also discovered in 2021
But I wouldn't expect a lot of entirely new types of subterranean mammals in any case. Most of the people would not bat an eye about another species of brown mouse. You need a theriologist for spotting the difference and they are getting more and more scarce.