A free-diver found a military canister bobbing in the waters off Hanauma Bay yesterday morning, forcing hundreds of people to abandon the marine preserve.

An Army explosive ordnance team was unable to detonate the device on the first try and had to be escorted by police back to Schofield Barracks to regroup for another attempt.

The team from the 706th Explosive Ordnance Disposal Battalion transported the phosphorus sea marker to a disposal area at Schofield Barracks around 5 p.m. and blew it up, the Army said in a statement last night.

Kurt Chambers was free-diving with a friend in the area known as Witches’ Brew around 9:30 a.m. when he saw the 3-foot-long, 6-inch-diameter cylinder and swam it into shore.

Chambers, a 30-year-old instructor with Freediving Instructors International, saw a warning label on the canister and worried that a child might play with it.

"It was floating, and with so many kids, anybody could have grabbed it," Chambers said. "This was a particularly large piece that was encrusted with algae. If it was fresh and clean, I would have thought differently (about bringing it to shore). … It did not appear to be a bomb, and I did not think of it as a bomb."

Chambers left the canister with a lifeguard, who called Hono­lulu firefighters.

The canister began smoking later as it lay near a lifeguard tower, said Hono­lulu Fire Capt. Earle Kea­loha.

Chambers was taking a shower at the park around 10 a.m. when he heard a public address announcement that Hanauma Bay was shutting down.

Around that time, the Honolulu police and fire departments contacted the 706th Battalion, which frequently works with HPD.

Police, firefighters and lifeguards stood by as the explosive ordnance team from Schofield Barracks arrived around 11:45 a.m., buried the cylinder and then tried to blow it up.

"Evidently it didn’t do the job it was supposed to do," Kea­loha said.

Honolulu police then escorted the ordnance team back to Schofield Barracks "to get more of what they need," Kea­loha said.

Hazardous materials experts from the Fire Department said they believe the device is a phosphorus canister used by Navy P-3 Orion pilots to mark objects, such as submarines, with white smoke in the water, Kea­loha said.

"We’re guessing it was used in a training exercise offshore and it floated in," Kea­loha said.

HFD hazardous materials officials recommended keeping people at least 100 feet away from the canister and leaving half of Hanauma Bay open to the public, Kea­loha said.

But as a precaution, Hanauma Bay officials cleared the entire marine preserve "for safety," Kea­loha said.

The bay is expected to be open today.