The archway to the tunnel is built out of limestone, while the tunnel itself is constructed out of brick.
The Machens are not sure what the tunnel was used for, but they do have some theories, believing it could have been a storage compartment for food or ice, the Telegraph reported.
The home, according to Gary Machens, was originally designed by architect Lucas Pfeiffenberger, the 25th mayor of Alton, the Telegraph reported.
The house was built in 1890, the tunnel is believed from 1840, so it was here for 50 years.