A huge explosion reverberated through downtown Bangor just after 3 p.m. on April 26, 1912, a century ago last week. A blast in an old brick reservoir under busy Mercantile Square left a crater 30 feet in diameter, unsealing a piece of the city’s infrastructure that most people had forgotten existed.

“Showers of paving stones, with here and there solid blocks of granite, were hurled a dozen feet into the air, and clouds of dust for a few minutes blotted out the scene,” recorded the Bangor Daily News the next morning.

A horse and wagon standing over the blast area were thrown 10 feet into the air, falling back into the reservoir, which contained about six feet of water. A man who had started to walk across the square found himself standing at the edge of the pit when the dust cleared, miraculously unscathed by the flying debris.

Squares were located strategically around the city. These open commercial spaces, usually located at street intersections, allowed men driving horses and wagons to maneuver easily with their cargoes. Mercantile Square was one of four such spaces linked together running roughly parallel between Main Street and the Kenduskeag Stream. In order, they were West Market Square, Mercantile Square, Pickering Square and Haymarket Square.

Much of the space that formed these squares is gone today and many of the buildings that stood around them have been torn down. West Market and Pickering still exist in attenuated versions along with their old names as public parks. Haymarket Square is a Key Bank parking lot.

Mercantile Square was located in the general vicinity of what is today One Merchants Plaza and, across the street, the Broad Street Market. It was one of the busiest spots in the city, “a favorite gathering place for baggage and furniture transfers, and at any hour of the day or evening the exact spot at which the explosion occurred has been covered with drivers, horses and wagons,” said the Bangor Daily News.

Miraculously, at the time of the explosion only the horse and wagon belonging to Charles B. Patterson, an expressman, were standing over the blast site. Patterson’s son Carl was in charge of the wagon. Just before the concussion he was standing nearby on the sidewalk near John T. Clark’s clothing store at 33 Mercantile Square. He had been planning in a few seconds to walk into the square and climb onto the wagon.

The horse and the badly smashed wagon landed in the reservoir about five feet below the surface of the street. Men rushed forward into the opening with ropes and a plank. They helped the “snorting, struggling animal” climb out of the hole nearly unscathed, and they brought out the wagon in pieces.

Meanwhile, frightened by the noise, “nearly a dozen horses started up Broad street on the run and almost a like number headed toward Pickering square and down Broad street. They were stopped by the police and bystanders without damage in each instance,” reported the Bangor Daily Commercial the next afternoon.

The fire department spent nearly half an hour pumping an estimated 10,000 gallons of water from the pit. Only then were officials sure no one had been killed.

The reservoir was part of a system of underground water storage areas built over the years all over the city for fighting fires. Both newspapers agreed that this particular one was built in 1865.

The Bangor Daily News said these reservoirs were “mostly forgotten and long unused.” The newspaper said the building of the city’s waterworks on the Penobscot River in 1875 “made them no longer necessary.”

The Commercial reported, however, that this particular reservoir had been used as recently as the great fire that destroyed much of the downtown a year ago. Indeed, both the fire and police chiefs said it should be restored.

The big question was what had caused the blast. Everyone seemed to agree gas had accumulated in the hole and exploded. But was it sewer gas or illuminating gas? The latter was produced by burning coal at the Bangor Gas Light Company on Main Street. This coal gas was used by people to light their homes and to power small stoves and water heaters. Underground gas lines ran throughout the city.

“I am very certain it was not illuminating gas,” C.R. Stull, general manager of the gas light company, told the Bangor Daily News for the first-day story. “We have two mains in the vicinity — one 20 feet from the reservoir and the other 50 feet. There was no leak from either. I am quite positive because workmen had been testing them a few hours before.”

City officials were skeptical of Stull’s claims. “Illuminating gas was the cause of the explosion,” stated Fire Chief Mason matter-of-factly. “I am very sure because when we arrived fire was coming out around the sides [of the hole].” Many people who worked in the area had smelled the gas. Indeed, gas company employees had been there trying to discover if there was such a leak.

Hearings before the city’s public utilities committee revealed a recent history of gas odors and testing for leaks in the area. For example, Henry C. Bean, an expressman “who stands on Mercantile Square, told of having seen two men digging in front of Fred Crowell’s produce establishment [at 47 Pickering Square] about 11 o’clock on the morning of the explosion. He said one of the men struck a match and flames spurted out of the hole.” The workmen were unable to find a leak, however. Other witnesses told of the strong smell of gas in their stores and in the air a day or two before the explosion.

A University of Maine chemistry professor, Ralph H. McKee, was brought in to do tests. One of his samples, taken from a hole in the pavement in front of Thurston & Kingsbury’s at 68 Broad St., contained enough illuminating gas to explode violently when mixed with air and ignited. Other samples showed no gas.

But the public utilities committee had heard enough. They concluded in May that responsibility for the blast lay with the gas light company. The next step was to take up negotiations with the company about payment to repair the reservoir and the square.

Wayne E. Reilly’s column on Bangor a century ago appears every other Monday. An illustrated collection, “Remembering Bangor: The Queen City Before the Great Fire,” is available where books are sold. Comments can be sent to him at wreilly.bdn@gmail.com.