I have noticed that almost all Mikvas are built into the ground (either as a basement or on the first floor). Why is that? I have heard that it could be an ease-of-use issue (one doesn't have to climb stairs), but if that was true, then there should be the same climb from a basement as the climb from the first floor?
They aren't all built near the ground - for example the mens mikvah where I live is on the second floor.
However they frequently build them on the ground simply because water is extremely heavy, and building it higher up requires special building reinforcement - the building would basically have to be built specifically to be a mikvah, and could not be renovated to a mikvah since it wouldn't be strong enough.
The standard design for a contemporary mikva has it fed by a system of gutters that channel rainwater directly into the mikva, without any elbows or other feature that could temporarily "hold" the water.
I'd figure this design would be simplified with a ground-level mikva.
Most swimming pools are also usually built into the ground. It seems that would be the easiest way for people to use them.