The FDNY has just shared a dramatic series of photographs from January 4th, 1969, showing the aftermath a massive gas explosion which "lifted 40‐to‐100‐foot sections" of Delancey Street in Manhattan.

The explosion occurred on the evening of January 3, 1969. According to a 1970 NTSB report, around 11 p.m., "a gas explosion erupted beneath a main downtown Manhattan Street, buckling the pavement for a distance of four blocks. The initial explosion was followed shortly by several more explosions, and fires along the path of the ruptured pavement. Three hundred families were evacuated from their homes. Fires continued to burn for more than 7 hours until the gas in the area could be shut off."

A New Yorker Talk of the Town piece from February 1st, 1969 described the aftermath: "Although the explosion created the customary inconveniences—services on two subway lines was suspended, bridge traffic was halted, and three hundred families were evacuated from nearby tenements—there were, miraculously, no deaths and only five minor injuries. (Two firemen were hit by flying debris, two Con Edison employees were blown off their feet by the force of the blast, and a girl seated in an automobile on Delancey Street was slammed headfirst into the windshield."

It turns out that there was an abandoned tunnel that had been trapped with leaked gas. At the time, FDNY Chief Fire Marshal Vincent Canty told the New Yorker, "When gas is confined in an unvented space, you get an explosion. And since this was a long, narrow explosion, extending for several blocks, there had to be some kind of long, narrow void underneath the road level. What puzzled us was that the fire seemed to be coming from an area between the subway, which runs down the center of Delancey Street, and the gas main, which runs along the north curb."

When the FDNY sent someone to the NYC Transit Authority, their blueprints revealed "there were two sealed-off tunnels"—"one on either side of the B.M.T. line," apparently built in 1908. The NTSB report of the following year delved further into the incident:

Although the blast occurred in a densely populated and heavily traveled district, only seven persons were injured in the accident. The explosion disrupted traffic and forced the closing of the westbound lane of [the Williamsburg Bridge]. Subway service was also disrupted.

About 11 hours before the explosion took place, a utility crew had been sent to the scene to repair a gas leak which was causing a small fire in the roadway. The fire went out and re-ignited by the repair crew several times. Workmen continued to search for the leak, and it was reported that the explosion occurred on one of the occasions the gas was being re-ignited.

After the accident, it was determined that an abandoned tunnel or "pipe gallery" was located beneath the roadway. This tunnel was constructed 60 years ago to hold utility ducts and pipes, but it was never used and subsequently sealed. Gas from leaking mains seeped into the tunnel, accumulating along its entire length, and was ignited, causing the blast.

The problem of locating the source of the gas leakage was complicated by the multiplicity of interconnected mains in the area. This condition stems back to when gas service was first provided in Manhattan. The area was supplied by a number of companies, each with its own gas main. Through the merger of these original companies, a condition of redundancy of parallel mains on the same streets, some of which are almost 100 years old, no exists. Because of this multiplicity of mains, it was difficult to pinpoint the leaking gas. This problem also exist throughout Manhattan. For example, there are 2,675,000 feet of street but almost 6,000,000 feet of gas main in Manhattan.

In 1971, the NY State Public Service Commission ordered Con Ed to improve its gas detecting program. (Some things never change.)

The tunnel under Delancey Street has since been filled in by the city.