A 35-year-old Florida blasting inspector is presumed dead after the ground at a mine collapsed Thursday, sending him plummeting into a lime rock pit, police said.

Kenneth James Stephens Jr. was working as a supervising contractor at Austin Powder Company as it detonated ammonia nitrate at the Mid Coast Aggregates mine in Webster, Florida, Sumter County Sheriff's Office Lt. Bobby Caruthers said by e-mail.

After the controlled explosion, the earth collapsed beneath Stephens when he was about 30 feet from the blast site. He fell into a water-filled pit about 100 yards across, Caruthers said.

Alerted shortly after noon, police arrived and initially saw no sign of the victim at the surface, nor did a police helicopter surveying the area from above. A crane lowered a pontoon, containing a dive team, into the pit to look for Stephens -- though, as of 6:30 p.m., his body had not been located.

Recovery personnel have begun the next step in their effort, building a berm, likely out of limestone, to seal off the area where Stephens fell from the rest of the pit. When that's complete, they will pump out the water in hopes of finding the victim's body.

Representatives from the Occupational and Safety Health Administration and the Mining Safety and Health Administration are on site, Caruthers said. He added that Austin Powder claims it has never had an accident like this since being founded in 1833, and that the sheriff's office has never responded to such an incident at this mine.

Stephens, a resident of Beverly Hills, Florida, had participated in more than 1,000 controlled blasts in his 11 years at the Austin Powder Company, said Caruthers.