Our Genex power station room is also underground.See power...

Our Genex power station room is also underground.

See power station "cavern" on this page
https://www.genexpower.com.au/project-details.html

For some complexy engineering reason, the actual turbines tend to be some meters lower than the lowest level of the lower reservoir. (hence I expect all to be in 'Caverns' (artificial or in the rock) to get blow lower reservoir water levels.

The lower height generates back pressure (hydrostaically) and that in turn make reducing cavitation in the turbines easier.

That ^^^ all TBMK I both not a hyrdo engineer nor a financuial adviser... but I read ... a lot.

Also see water hammer and how building *in* bedrock probably isn't a bad idea as it will help make turn the tap off easier and safer.
Less likely to blow the 100T turbine out of the ground like a bullet. Also gives you somewhere to build the surge tank/shaft that is used to cope with water hammer.