So what was the Turtle Lair originally intended for?

[neatoman]

So why is there an abandoned apartment sized chamber with electrical output in a sewer?
No that's a serious question, what's an unused yet livable chamber doing in a sewer? What was such space intended to be used for?

I know there are some variants that make a little bit more sense (basement level apartment, mystical outpost, old pumpstation, bomb shelter, etc) but quite often it's just some unexplained chamber that just happens to be located in the sewers, bizarre huh?