Something mysterious happened at a packed beach in Narragansett, R.I., Saturday that caused the ground to shake, produced a loud "boom," and sent a woman to the hospital in what was widely called an explosion.
But authorities said Sunday the investigation does not suggest there was an explosion at Salty Brine State Beach, and they are considering whether the cause could be a natural phenomenon.
"We're still looking into it," said Lieutenant Colonel Todd Catlow of the Rhode Island State Police Sunday. "There's no evidence of an incendiary device, there's no residue, and there was no smoke."
Investigators are eyeing the possibility that the blast was caused by a "geological movement."
"We're looking at everything we can," Catlow said. "It's not a normal investigation."
The Rhode Island Department of Environmental Management and the state fire marshal's office determined the beach was safe to open to the public Sunday, said Rayna Maguire, assistant to the director of the Department of Environmental Management .
Wayne Stepalavich, who was at the beach with his family, was about 100 feet away from the mysterious blast, he wrote on Twitter. Stepalavich did not immediately return calls for comment Sunday.
"Woman was seen 'flying,' " he tweeted. "It was loud and shook the ground."
"[It] was very surreal," he continued. "There was not a lot of panic, just confusion. Everyone was calling 911."
A woman was blown over by the blast, authorities said, and she was transported to South County Hospital.
Larry Mouradjian, associate director of natural resources at the state Department of Environmental Management, said Saturday the blast, which occurred below the high-tide line on the shore, created fissures in the ground.
As authorities continue to investigate the puzzling occurrence, Daniel Sheehy, president of Aquabio, Inc., an environmental consulting services firm in Arlington, speculated that decomposed seaweed could be to blame. "When seaweed decomposes, methane and nitrogen are released," Sheehy said. "Hydrogen sulfide has a rotten egg smell. At elevated concentration, it's toxic and also flammable."
Sheehy said that if the seaweed is buried, it might cause methane to build up. Some beaches bury seaweed to get rid of it, he said.
John E. Ebel, senior research scientist at the Weston Observatory and professor of geophysics at Boston College, said the explosion has him stumped.
A mix of oil and natural gas could cause an explosion, but conditions are not right for that to happen in Rhode Island.
The state does not have the geological environment that would produce ice known as methane hydrates or heated rocks like the kind found around active volcanoes, both of which could cause a blast like the one on Salty Brine Beach, he said.
"I cannot come up with any good natural explanation," Ebel said. "I'm thinking through all of the lectures, the talks. . . . All of the natural explanations that I could think of in other parts of the world don't apply to Rhode Island."