Beneath the Brutalist structure of Boston City Hall is a dusty abandoned tunnel that was once a part of America’s first subway line, the Tremont Street Subway.

On May 21, in honor of Preservation Month, 100 people will have the rare opportunity to tour the cavernous space, which was in operation from the end of the 19th century to the early 1960s.

The Boston Landmarks Commission and city’s Archaeology Program worked for several months to make the tours happen, but city archeologist Joe Bagley said they never expected such an outpouring of interest.

Bagley posted the free events on social media at noon on Tuesday, and within half an hour, all five 20-minute tours were full. Over the next 24 hours, thousands more clicked on the link.

“We expected it to be popular, but this kind of blew us all away,’’ Bagley said. “There’s a lot of demand and now we have to figure out how to accommodate it.’’

So, fear not, aspiring urban explorers. Officials are already working on hosting more tours in the near future.

“This is one of the first tunnels ever built for a subway, period,’’ Bagley said. “It’s also connecting to a station that doesn’t exist anymore. There used to be a station called Adams Square, which was in the middle of a former portion of Washington Street where City Hall is now.’’

The tunnel opened in 1898 as an extension of the Tremont Street Subway, which had its first run the year before, between Boylston and Park Street Stations. It was the first subway system built in the United States.

The abandoned tunnel spanned from Scollay Square to Adams Square along what used to be Cornhill Street. Unlike an abandoned subway in New York City that has colorful mosaics, this is a fairly simple space. There’s an antechamber and then a door, which gives way to a vast interior that doesn’t contain much except some bits of rusty metal and an old chair.

It was abandoned in the early ’60s as part of the redevelopment of Government Center.

For the tours, organizers plan to place a row of lights inside the empty tunnel so it will be well-lit for those who want to take pictures. Ticketed visitors are asked to wear sturdy shoes as it’s plenty messy down there.

“The ground is dirt,’’ Bagley said. “You can still see the shape of the tunnel and the structures of the support roof and these little people-shaped nooks down the side where you could tuck yourself in if a train was going by.’’

Cristela Guerra can be reached at cristela.guerra@globe.com.