I want there to be an Underdark, but the ecosystem can't work because of energy from the sun. Except maybe it can.
Go and read this interview with Tullis Onstott.
The rest of this post is just zero-creativity rip of quotes from the same thing for people who don't have the time to read it. You should thank Discover Magazine, not me.
"And it had genes for chemoreception, which tells us it’s sensing
has the capability of moving around. The idea that organisms down there
might be moving around and interacting with the environment—that was
really surprising. The only tip-off from the genome that this is a
subsurface organism is that it has no protection against oxygen. As soon
as it hits air, it’s dead."
Yesssss, chromoreceptors two miles down, sensing something, moving around, yesssss.
"Candidatus Desulforudis audaxviator is entirely
self-sufficient. It has its energy source, radiation..... Such things aren’t supposed to exist."
Feeding on radiation in the lightless depths, good, goooood.
"There may exist a shadow biology—very, very primitive organisms that may
have come into existence very early on our planet but were completely
replaced by DNA organisms everywhere else."
"The mine has a helical tunnel that goes a kilometer and a half down. All
this warm air comes up from below, and as soon as it hits the
permafrost layer, where the ground is permanently frozen, all the
moisture in the air crystallizes and you get huge snowflakes, a couple
of feet wide"
"The more I learn, the more it seems that the requirements for life are
pretty minimal. The niches that life can occupy never cease to amaze me.
A place may look terrible to us, but to something else, that’s their
There is also this:- "Subsurface biota extends over a wide variety of habitats that can be spatially interconnected."
(Click image for link)