Tiber

Tiber presents as steel-grey blue, with calm surface waters concealing immense crushing depths, with two moons — the larger on an outer orbit, the smaller tracing a closer, faster path. Surface temperatures range from -2 to 40°C at over 3,000 atm, with evaporation storms feeding the cloud canopy for centuries at a time. At 7,124 km across, the surface material is primarily a cryogenic ocean of liquid methane over rocky sediment. Whatever is happening at the seafloor of Tiber is happening in complete isolation from the surface — and has been for a very long time.