underground: usa was a 12-week documentary road trip. marshall took it between june and september 2009 where he went to all 48 mainland states of the us and in each one visited an area that shared a name with a station on the tube map, for example, epping, maine, where the journey started. he had his filming equipment stolen during the trip but eventually turned the story into a one-hour youtube documentary as well as publishing a related book "underground: usa".