(UPI) An Illinois couple who moved into their home in December made a surprising discovery while repairing their sidewalk -- a 19th-century tunnel underneath the house.

Gary and Beth Machens said they moved into their Alton home in December and recently decided to repair a section of sidewalk that had started to slope.

Gary Machens said he had to remove part of a stone wall on the sidewalk to make the repairs, and he then discovered the entryway to a tunnel.

"He called me at work and said, 'You better get home; you're not going to believe what I just found,'" Beth Machens told the Alton Telegraph.

The couple said the tunnel is about 9 feet high and at least 60 feet deep.

Local historians estimated the tunnel was created in 1840, 50 years before the house was built, but they do not know the original purpose of the underground structure.

"It's unique. There are a few tunnels around the Alton area" according to the Landmark Historic Society, Gary Machen told KTVI-TV. It's been used as an icehouse or root cellar or a lot of other options.

"It could have been used for the Underground Railroad. There's no proof of that, but there was a ferry here in the Alton area to the Missouri side, and it's possible it could have been used for that," he said.

Machen said he hopes to preserve the tunnel and possibly give tours if it is deemed safe and the city agrees to help with the cost.

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