No not thunder DOME, Thunder DROME.
Last year a guy decided that he wanted to cut the grass in some of the run down parks in the City of Detroit. He had a riding lawn mower and just wanted a place to do it. He started a group called the Mower Gang. In one of the parks he discovered an old abandoned velodrome (bike racing track). It is about an 1/8 mile cement oval.
He and some of his friends cleaned it up and some others decided to ride motorcycles on it. One thing led to another and now there are a whole series of underground moped, scooter, pit bike, minibike, go cart and even bicycle races on it. I've ridden on it myself on both my moped and Honda CT90 though I have not raced. It is a bit scary but a lot of fun. The big race is coming up this weekend.
Here is the link.