In the basement of Jo’s Nautical Bar, there is a watertight door which is an entrance to the Boston Harbor Tunnel System. The building at 125 Main Street, which is now Jo’s, was a control center for the tunnel system from 1922 till 1952. The door is opened occasionally for tours, and hikers can walk from Hull Gut all the way to Deer Island and several other spots on the harbor.
The Boston Harbor tunnel system is an outgrowth of two earlier networks of tunnels under the harbor which reached from Deer Island to Scituate Harbor. The first tunnels were dug in 1764, from Fort Independence to Fort Warren. Military engineer Sylvanias Thayer greatly extended the system in the early 1800’s to a system of 58 miles of tunnels with many large underground structures. One of these structures included a barracks for 2,000 troops and was connected to Long Island by a mule-driven narrow gauge railway.
The system got its second major revamp in 1894 when many of the existing harbor fortifications were built or redone. The huge barracks (194 feet below the 23 foot deep bottom) was retained, but 22 miles of the network were sealed off and not maintained. President Hoover toured the tunnels in 1929, walking from Deer Island to Fort Strong on Long Island.
In 1943 the network of tunnels was upgraded, though 5 miles of the older network were sealed off. Now only Great Brewster, Little Brewster (Boston Light), Deer Island, Long Island, and Peddocks Island (two entrances) are served, plus Hull Gut and Weymouth Fore River on the mainland. In recent years tours of the tunnels have been held in April and December (a Christmas eve walk to Boston Light on Little Brewster by Santa and many followers).