Portable ground penetrating radars are becoming a very common sight. Units such as this one can often be seen used by construction surveyors: http://www.geophysical.com/Images/GPRfigure2.jpg . These machines are expensive but it is likely they can be rented for a day or two at a reasonable price. The biggest problem is finding somebody experienced in using this technology who is local to DC area and would be willing to sacrifice a couple of hours of his/her time scanning the sidewalks in the area around Comet Ping Pong, Besta Pizza, etc. (preferably very early in the morning when there are few people around on the streets). Obviously, the person should be proficient enough to be able to tell definitively what he sees on the screen, i.e. a water main, a sewage pipe or an actual tunnel. Thanks and greetings from Canada!