An easily overlooked sign on the banks of the Potomac River beside the Department of Defense warns mariners in no uncertain terms: “CABLE CROSSING DO NOT ANCHOR.”

The bluntly worded signage marks an important if obscure bit of national security infrastructure: the Pentagon’s underwater data and telephone connection to Washington, D.C.

The cables underneath the Potomac River date to the early 1940s as construction of the Pentagon was nearing completion at a blistering around-the-clock pace. A 1943 article in Popular Science describes how 12 submarine telephone cables were paid out by two specially equipped barges to service “the world’s largest private branch switchboard, daily put through a total of about 200,000 outgoing and incoming calls,” plus another 100,000 internal calls.

The submarine connection was designed by Bell Telephone and Western Electric engineers to link the then-remote Pentagon with the national telephone system across the river in D.C. Military divers at the time made an effort to nudge the submarine cables into a 2,000-foot-long underwater trench, but they seemed to have snagged onto a problem within just a few years.

In 1949 newspapers across the country reported that “the anchor [of a Potomac tug boat has] snagged a fat cable - one of the most complicated in the world - that carries all of our war machine’s communications with its bosses and peers, the War and Navy Department’s connections to their commander in chief and all the various departments of the executive government in Washington. There was a mighty snap, and forthwith world’s mightiest war machine was cut off from the spendingest [sic] and most  wide-world-spread government on earth - and vice versa!”

Repair crews quickly jumped into motion laying down an emergency cable, but throughout the day “generals and admirals and civilian department heads began sending clerks to coin box telephones with important messages and government-asked confidential information.”

Based on the location of the modern signage, erected by Verizon, the Pentagon still appears to use the submarine connection, or at least an updated set of cables within the old trench. This information will come as no surprise to anyone who has called the federal government’s Pentagonal dinosaur home. The building’s archaic technology and offices are the stuff of legends, and in one telling anecdote, the DoD reportedly still maintains contract management software built on punchcards.