First discovered inside Jupiter's moons, underground oceans are very common in the universe.
Ten times deeper than Earth's oceans, the pressure at the bottom is enormous. In the oldest and coldest ones, water molecules separate into layers according to isotope ratios. Deep drills extract plentiful fusion fuel.
The chemical segregation is even more interesting. Near the top are dissolved gases, at the bottom organic molecules.
Linking these regions can unleash abundant energy.
Many bacteria that evolved in the subsurface oceans have folding outer membranes that expand or reduce their cell volume, to float or sink as needed.
Long-chain polymers drape tens of kilometers from the ice ceiling. Strange traffic flows along these organic stalactites.
To exploit the ocean's heat gradient, the most advanced organisms have to be very big.

Reconstructed transmission from submarine probe 956 at Torobruk-59:
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. . . new paradigm
. . . given time each world ocean will evolve into a single cell
. . . free-floating genetic code repeated everywhere . . . activated as needed
. . . realized the ocean is one immense being
. . . floating crystals at -50 km
. . . working memory stored in the nutrient gradients around ionic clusters are the source of the aperiodic flashing
. . . probably doesn't know we're here



The best hard SF novel ever written: Infinite Thunder by Jack Arcalon.
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