A crumbling network of old utility tunnels under Capitol Hill connect 24 government buildings to the Capitol Power Plant. The original tunnels (marked below with yellow dashes) were excavated in 1908 to supply steam and electricity to new congressional office buildings and the recently completed Library of Congress. 

Running along First Street, the utility corridors passed over the Washington Terminal Company's railroad tunnel and under the Library of Congress book tunnel. The dimensions were cramped and swampy: tunnel workers shared the 7 foot tall, 4.5 foot wide space with steam pipes that raised temperatures over 100 degrees.

The steam tunnels were later expanded to the north to link up with Union Station, the Government Printing Office, and the old Post Office. 

Construction on top of the First Street railroad tunnel was problematic, and in 1949 the Washington Post wrote that "vibration from the trains below, and traffic above, has caused the tunnel to settle unevenly from 18 to 24 inches."

The original steam tunnel branch was abandoned in place in 1950 following construction of a parallel tunnel under Second Street. These newer passages were slightly larger, at 12 feet by 10 feet, but the temperature still hovered at around 90 degrees. Worse still - they were insulated throughout with asbestos.

In 2006 the 10-man AOC tunnel crew made headlines with complaints about workplace danger and crumbling concrete walls. An NBC video from the time provides a rare glimpse into the steam tunnels. This set into motion a long overdue effort to repair the tunnels that was completed in 2012.

Screengrabs from the NBC video are included below: