The barnacle-encrusted bomb rested idly in the lapping Pass-a-Grille surf, looking more like an old log than a piece of military equipment.
But a beachgoer walking along the water's edge about 8:40 a.m. Sunday took a closer look at the 4-foot cylinder and decided to call authorities.
That call triggered a daylong effort to evacuate beachgoers and homeowners, build a sand berm around the bomb, protect hatching sea turtles and safely detonate the M122 World War II-era flare where it was found.
After the walker reported the object near the 22nd Avenue beach access, Pinellas County sheriff's deputies visited the area. They said the cylinder "appeared to have been submerged for a significant period of time."
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They cleared a swath of beach around the bomb, shifting visitors about 900 feet on each side "in an abundance of caution."
By Sunday evening, the perimeter had expanded to 1,400 feet on each side, encompassing the blocks from 18th to about 30th avenues.
Next, the Hillsborough County Sheriff's Office bomb squad took a look at the object, which deputies said was a military ordnance, a term for weapons or ammunition. The bomb squad turned to MacDill Air Force Base's explosive ordnance disposal team for further assistance.
Dozens of rapt visitors sat in folding chairs behind police tape staked in the sand. St. Pete Beach Mayor Maria Lowe updated them as authorities used a bulldozer to build protective berms around the sea turtle nests and the bomb, and planned for detonation using C-4. Three nests were located in the evacuated area, one within 10 feet of the bomb site.
They peppered Lowe with questions: Is the bomb old? What about the turtles? Can we have a beach party after this?
Lowe said about 25 homes were evacuated and 250 beachgoers displaced, but most were in good spirits, amused to be part of such a strange story.
"Nothing ever happens out here. It's so crazy," said Katie Hellier, 49, of Pass-a-Grille, who stood on the beach with family.
Best friends Bridgette Treadwell and Courtney Smith, taking a kid-free vacation from the Memphis area, had been hoping for an uninterrupted beach day after a spell of rain. But they had fun with the commotion, sharing a pair of binoculars to get a glimpse of the faraway action.
"Our kids would be getting a kick out of this," Smith said.
Michael and Denise Taylor, who live in St. Pete Beach, spent the whole day shuffling farther down the sand as authorities kept expanding the cleared area.
"We were all joking about it, except for a few," Denise Taylor said. She said she had talked to a young woman who saw the bomb. "She walked by it earlier and she honestly thought it was a log."
The bomb was used to create bright, intense light to aid WWII-era night photographic missions, according to a 1957 explosives guide by the U.S. Navy.
It weighed 103 pounds, had a candlepower of 45 million and boasted a flash so bright it was "detrimental to vision to watch the explosion."
Authorities chose to detonate the bomb because of the potential for a dangerous explosion or fire, said Lt. Patrick Gargan, spokesman for the 6th Air Mobility Wing at MacDill.
"The intent of the detonation is to completely destroy the device itself," Lowe said. Since the bomb is not shrapnel-style, she said, it would have a relatively clean explosion.
The nonprofit group Sea Turtle Trackers was on site to monitor the turtle nests, she said. After the explosion, the group told her the nests were not physically touched, but until hatching season ends, it will remain unclear whether the nests were disrupted.
"I hope you've had a great day at the beach with a little excitement," Lowe said to beachgoers with a laugh as she announced a time for the explosion. "I don't know if I'm going to have the opportunity to do a countdown, so please do not be taken off guard if you hear a large boom."
A little after 5 p.m., a bolt of black and gray smoke burst from the ground, followed by a dull cannon thud and the screeching of seagulls wheeling away from the blast. The crowd yipped and clapped and then, as white smoke was still drifting over the dunes, began to scatter. Beachgoers shook out towels, folded up umbrellas and wheeled coolers up the boardwalks to their cars.
The show was over.