Boston Harbor's Controlled Mines: Defending the Channels from Shore

For half a century, the channels into Boston Harbor were sown with explosives that could be fired from shore at the press of a switch. These controlled submarine mines, watched over by soldiers on the harbor islands, were part of the country's answer to a problem the Civil War had exposed: stone forts could no longer stop a modern warship.

Why the harbor needed mines

After the Civil War, warships grew more advanced and rifled guns could smash the stone walls of the Third System forts. An effort in the 1870s to add gun batteries, heavy mortars, mines and channel obstructions mostly stalled. In 1885 President Grover Cleveland convened a joint Navy–civilian board to start again. The Endicott Board's recommendations included floating batteries, torpedo boats — and submarine mines.

Boston's Endicott-era defenses

Mine facilities were built in harbors across the country from the 1880s, but no mines were actually laid until the Spanish–American War in 1898. In time the Army mined 28 major U.S. harbors.

How a controlled mine worked

• The mine: a watertight case holding the explosive, and a firing device wired to shore.
• Ground mines sat on the seabed and could be set off by the control board on shore, by contact, or by a ship's magnetism. Because soldiers on the islands could fire them, they were controlled mines — safe for friendly ships, deadly on command.
• Laying them took a small fleet: a mine planter (purpose-built after 1904, with wide decks for assembled mines), a distribution boat carrying the distribution box that every mine in a group was cabled to, and two to four yawls to ferry ropes and cables. Every power and circuit connection was tested first.
• Keeping them meant regular checks and maintenance once they were down.

Mines in Boston Harbor, 1945. The triangles are groups of mines controlled from Georges Island, Great Brewster Island and Deer Island. U.S. Army Engineers, "Supplement to the Harbor Defense Project: Harbor Defenses of Boston," January 31, 1945

Two world wars

In World War I the islands' soldiers ran the minefields, and in July 1918 the system went on alert — a sign that a German U-boat might have entered the harbor. Nothing came of it.

By World War II the Army had fortified nine islands, and the minefield was a shared operation:

A new control apparatus for the ground mines went in in 1943. Because the mines could be held safe for friendly ships, the harbor stayed open to its own traffic all through the war.

Afterwards

When operations ended after World War II, the government sold many of the islands as surplus. Many became part of Boston Harbor Islands State Park — today Boston Harbor Islands National and State Park.

Sources

Written from "Controlled Submarine Mines in Boston Harbor," Boston Harbor Islands National Recreation Area, National Park Service; a work of the United States government in the public domain. It draws on the Olmsted Center's Cultural Landscape Report: Boston Harbor Islands (2017), Mark Berhow's American Seacoast Defenses (2020), Gerald W. Butler's The Guns of Boston Harbor (2001), and Shurcliff & Merrill's History and Master Plan, Georges Island and Fort Warren (1960).