It's been several days since a mysterious explosion propelled a beachgoer 10 feet into the air at Salty Brine Beach in Narragansett, Rhode Island, and authorities still have no idea why.
While speculation as to the cause of Saturday's incident includes a methane explosion caused by decayed seaweed or other organic matter under the sand, Stephen Porder, associate professor of ecology and evolutionary biology at Brown University, told the Associated Press, "Everything at this point is entirely speculation."
The beachgoer, Kathleen Danise, was thrown from her beach chair by the mysterious explosion, sustaining a concussion and a pair of fractured ribs when she was slammed into the rocks.
#SaltyBrine beach explosion victim 60y/o Kathleen Danise is a registered nurse, mom & grandma; sister says @wpri12 pic.twitter.com/5CvzKwDH9l— Chantee Lans (@ChanteeLans) July 13, 2015
Witnesses have said that they heard a rumbling and a loud bang that sounded like a firecracker, a grenade or a gas explosion, then noticed a sulfur or butane smell. The blast left behind a "rift" in the sand -- but little else.
Porder offered that methane might have built up from organic matter trapped underground when the jetty was constructed.
"You can definitely get an explosion. How you would have that sequence of events on that particular beach on that particular day, I don't know," Porder said. He also pointed out that it is typically places like swamps or areas in the arctic that produce high levels of methane.
"I've never heard of a methane explosion on the beach," he added.
Sun sets over #RI #SaltyBrine beach where an explosion injured a woman. Details on investigation @wpri12 at 10/11! pic.twitter.com/9cdxKcUckV— Chantee Lans (@ChanteeLans) July 12, 2015
State Police Col. Steven G. O'Donnell said there was no physical evidence of an explosion, such as charring or debris. Dogs and chemical swipes detected no explosive residue, either. Officials also determined there was no natural gas line running underneath.
In addition, a "malicious attack" was ruled out.
#RI DEM: "There continues to be no evidence of a malicious attack or use of a device of any kind. #SaltyBrine beach remains open." @wpri12— Chantee Lans (@ChanteeLans) July 14, 2015
As to the suggestion of a gas leak, authorities noted the nearest line was at least 100 feet from where the explosion took place. And late Monday, investigators with the Department of Environmental Management (DEM) noted that there were no records of a seismic event in Rhode Island, canceling out one of the propose theories.
According to a @UConn seismologist, there were no seismic events that registered in #RI during #SaltyBrine beach explosion; DEM says @wpri12— Chantee Lans (@ChanteeLans) July 14, 2015
Also on Monday, a cable used by the U.S. Coast Guard to power the navigational light at the end of the jetty, was dug up from the sand. A DEM spokeswoman said the cable was no longer live and there were no indications it was related to the blast. All the same, Coast Guard officials revisited the cable Tuesday, digging parts of it out of the beach to inspect it.
Crews are working to dig up the cable discovered at #SaltyBrine beach. Area closed to the public. @ABC6 pic.twitter.com/ogBqW6uKrb— Melissa Randall (@MRandallABC6) July 14, 2015
Part of the cable taped and wrapped up at #SaltyBrine Beach @wpri12 pic.twitter.com/HtcZsyZyPj— Susan Campbell (@S_E_Campbell) July 14, 2015
Rob Thieler, a marine and coastal geologist with the U.S. Geological Survey, had another theory about a possible source of methane: estuary sediments, which sometimes cause explosions when they decay. He said scientists have seen evidence of this phenomenon off the coast of Maine.
"The sea-floor surface looks like a golf ball, and these are presumably places where underlying methane produced by estuary sediment has violently escaped," Thieler said.
This type of explosion could happen spontaneously, he said, though having it happen on a sandy beach is rare.
Despite the explosion, visitors to the beach weren't scared away.
Still a decent crowd here today. #SaltyBrine @ABC6 pic.twitter.com/D40VDvaD6y— Melissa Randall (@MRandallABC6) July 14, 2015
Additional reporting by The Associated Press