Well, I always wondered if we could simply melt large tunnels, and cathedral sized caverns into the thick ice of Europa!

If so, then that seems like a pretty quick way to build city sized spaces (consisting of caverns and tunnels) in the ice, complete with lots of lakes and ponds, where-ever you wanted them.

So... beneath the first colony ships/buildings on the surface of Europa, you could have access port-hatchways, to the tunnels, and then pressurize those tunnels/caverns with air, giving humans a lot of room to frolic and run around.

SIDE NOTE:

As a side note, I remember reading a recent sci-fi story in which it was suggested that you could not touch the ice of those outer moons with your bare hands, since the ice is so cold.

In that sci-fi story, your hand would instantly freeze-weld itself to the ice, and the heat would be rather quickly "sucked" from your entire body, starting with your hand, moving to your arm, and then the rest of you.

If that is the case with Europa ice, then we would need to insulate humans, animals, and plants from touching the raw-ice (perhaps with some kind of thin material).

NEXT... I guess you could put up a vast array of solar light collecting spherical-domes on the surface, that would simply gather, concentrate, and pipe sunlight into the tunnels and caverns with fiber optic cables.

That would light the entire place up cheaply, and perhaps you could then try to grow a vast array of planets, and also create ponds/lakes for fish.

But I imagine the big problem for life in these Europa ice caves and tunnels: SOIL and BIO-MASS!

On Mars we can make our own bio-mass and soil, by gradually mixing Earth bacteria/soil, with Martian dirt/regolith.

So for example, on Mars, you could start with half Earth soil, half Martian regolith (or whatever the optimal ratio), and MIX them together, spreading the earth bacteria all throughout the mix, creating a HYBRID-SOIL.

I'm guessing the martian dirt/regolith would also add a fair amount of trace elements necessary to support life.

Plants would then grow in this hybrid-soil. Humans and animals would eat the plants and poop. You would then mix the poop and other dead plant material back into your hybrid-soil, and add another dash of Martian dirt.

Gradually you would expand like that, getting more and more vast amounts hybrid soil.

BUT... Europa doesn't have much accessible dirt/regolith on it's ice surface (I think)... perhaps maybe a few chunks of meteors that we could grind up?

So without that dirt/regolith to add bulk and basic elements needed by life, I'm not sure how far your total biomass could get.

I'm guessing you would have to import, then grind up rocks from nearby smaller moons, to make your own regolith.

If the nearby moons are small enough, you won't use up too much fuel lifting, and flying the rocks back to Europa. (And of course you could make the rocket fuel, to help support this effort, from Europa's water by splitting the water's hydrogen/oxygen).