Good question. I'm no spelunker, but I imagine the ground somewhere down there would be permeable. Also, there might be a possibility that it would be fresh water, wouldn't there? Or saltwater.
I can't imagine pumping ocean water through any big equipment due to the corrosive nature.
All I can surmise about the algal bloom is that something is coming out those drain pipes that probably isn't ocean water.
I've archived a LOT but I don't know if I archived anon's conversations on the island. I'll have to really dig to find that info again.
That is the thing, it's a small island so the whole environment would be salty and humid. Maintaining creature comforts would be a full time job for at least a few people. You would need good people, spare parts and regular maintenance. Sure, you can automate and monitor remotely, but when things fail, you need hands on to get things going again.
My assumption is that the "floor level" of the underground complex was well above sea level (~30' AMSL). My guess is that any water in the system may have been rain seepage from above.
I can see three likely drainage points -- but, there may well have been electrical pumps inside the system to move water to those points...
TXnMA
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AHA!!! The power of multiple minds!
aquamarine reminded me that underground spaces are typically several degrees colder than aboveground air ambient temp -- so -- the A/C function for the tunnel/underground room complex may well have been one of heating instead of cooling.
Then, reviewing the drainage issue reminded me that another A/C function is Humidity control. On a semi-tropical island, humidity is a big deal! Which means that large volumes of (essentially) pure, distilled water are removed -- and must be disposed of...
I recognize that seawater in the USVI is already pretty warm -- but, what would a steady flow of (even warmer) fresh water (dehumidifier discharge) into the ocean shallows -- do for algae growth?
ThanQ to all! TXnMA