There was a NASA proposal back in 2003 to eventually (2040s) send an expedition to Callisto, the outermost Galilean moon of Jupiter, and turn it into a major space base. Details on that in a bit.

A Crazy Idea: Infinite Hydroelectric Power

Callisto is thought to have a subsurface saltwater ocean below the 80-150 km thick crust. If this proves true, and if there are no worthwhile organisms inhabiting this underground worldwide sea, then:

1. Cut a hole through the crust, which might be of low density as the whole world of Callisto is of a lower density than the other Galilean moons;

2. Lower a huge hydroelectric turbine, with power cables and a mount to attach it to the underside of the crust;

3. Turn it on.

The subsurface ocean might be stirred constantly by a tidal-gravity-based combination of Callisto's orbit around Jupiter interacting with the gravity of the other Galilean satellites. It seems logical to place the hydroelectric plant along Callisto's equator or thereabouts to get the maximum effect from being in roughly the same orbital plane as the other satellites and of course Jupiter, taking advantage of age-old currents. I cannot believe the ocean there would be stagnant, but who knows.

Problems

1. The subsurface ocean is sure to be corrosive, so the hydroelectric turbine(s) will need constant replacement and repair.

2. The turbine will produce a bit of drag on the subsurface ocean, which might affect the orbit of Callisto around Jupiter.

3. The turbine(s) will certainly produce heat, slowly warming up the underground ocean with unknown effects.