Roller Coaster Tycoon 1 underground bathroom

In the original release of RCT1, this is not possible as you can only build tracked rides and pathways underground, and that is if they start above-ground first (you can start building above-ground and then remove the above-ground track once the ride goes underground. Attractions such as shops, toilets and single-piece gentle rides cannot be placed underground for that reason.

In RCT2 and the Android re-release of RCT, you can move an attraction underground using the shift key and the onscreen arrow controls respectively.

It's been a while, but in two of the scenarios in base RCT there are completely underground rides - rollercoasters specifically. I don't think placing non-tracked rides like the merry go round or shops was allowed.

To achieve this you build your ride and then lower the ground underneath causing it to "close over". That's what I remember at least, doing things underground could be a pain at best.