Case in point, just last week city workers stumbled upon a cache of survival supplies in the the Manhattan anchorage dating back to the 1950s and 60s. Nobody knows their exact purpose or how they got there, although it seems the survivalist cracker rations and anti-shock drugs would have been thought useful in the case of nuclear attack. The area could not be used as a fallout shelter, but the find is intriguing and very cool - the crackers even still seem intact!
So, even the city workers don't know what's inside the bridge bases (then again, I suppose that should be no great surprise). I heard that the Brooklyn anchorage was cleared out some years ago and used as a performance space. But who knows what-all has been in the cavernous areas below the bridge over the years? Bridge parts? Old cable trains? Mole people?
I haven't been able to turn anything up yet, although I seem to recall that there may have once been market stalls on the Manhattan side. I'm also curious as to whether the Brooklyn anchorage is still open to the public in some capacity, but it seems it will take more research to find out.
So, if anyone has any answers to these questions, I'd love to hear them. If, on the other hand, you know nothing about the remarkable history of this structure, check out this site chock full of old articles, engravings and photos. The pictures of the old train terminals alone are enough to get a certain NYC history geek drooling.