In Washington DC, underneath Dupont Circle, there is some interesting unused space (approx 26,000 square feet of it). It was originally a trolley subway, but it was closed down in 1962. It briefly reopened in 1995 as a food court, and by "briefly" I mean the food court was open for 6 months.
Apparently there's a move afoot to put the space to use again, this time as an art space. It'll be interesting to see how that works out.
I just think it's fascinating that an unused underground facility exists in downtown DC, neglected but available for development. I hope someone figures out how to take advantage of it and turn it into something productive.
It makes me wonder how many other underutilized resources are out there, waiting for someone to apply creativity, imagination and vision (along with some dollars and elbow-grease). Just a thought...