Engineeringfag here, sorry if this has already been discussed.
My thoughts on this building are that it's a multi use engine house.
1) the ~12" exhaust stacks makes me believe there's 2 1MW marine grade gensets
2) the external piping is for HVAC into the tunnel system
3) the engines pull a vacuum through their intakes
4) the engines are cooled by sea water
5) at part load the engine's exhaust is run through the water for muffling purposes. which could explain the algae bloom
6) 2 underground fuel tanks like at gas stations
There could be additional machinery for HVAC purposes as well as a backup generator. The man sized door is to the control room and switchgear access.