In the treacherous waters around the I-35W bridge collapse site, divers navigate a minefield of submerged metal and concrete.
"The current could pin you against something or impale you on something," said Capt. Bill Chandler of Minnesota's Hennepin County Sheriff's Department. "These are very stressful diving conditions. It takes a lot of mental stamina, physical stamina, and they have to pay attention."
Making matters worse, divers can barely see the hazards that they're trying to avoid. With visibility measured in inches, a mere 14 feet of murky, oily water (the depth of the Mississippi River at the site) can seem like an abyss.
"It's like a fog down there," said Hennepin County Sheriff Rich Stanek. "You've got silt. You've got stuff moving around, debris. We're moving around causing more."
ABC News traveled upriver to see firsthand the challenges the Mississippi poses, enlisting the help of recovery diver Bill Matthies, who's been pulling accident victims from the river for 50 years.
"Your hands are your eyes," said Matthies. "That's how you see."
Dressed in full scuba gear, Matthies demonstrated how people and objects -- even those a few feet away -- simply disappear underwater in a haze of sediment and algae. Visibility on a clear day can be 2 to 3 feet. On rainy days, it can be zero.
Matthies said it's easy for a diver to get lost in a maze of wreckage at the collapse site.
"I'd be worried about swimming underneath something and not knowing what's up there, and then when you want to come up, then you're caught," said Matthies.
Authorities said one of the I-35W divers Saturday got briefly tangled in debris. He was eventually able to get free with the help of his backup team.
Once a diver manages to get to a crumpled vehicle, the risk of injury multiplies.
Florida dive expert Pete Gannon showed ABC News how recovery divers scan for bodies by feeling around the interiors of vehicles, ships or other sunken structures. They must be careful not to unsettle already-tenuous piles of debris or cut themselves on jagged edges.
"It's extremely dangerous," said Gannon. "Eighty-three percent of diver fatalities happen in training and recovery operations."
Before the I-35W dive effort, there had been underwater operations of larger scope -- including the salvage mission following the 1996 crash of TWA flight 800 -- but few have been as dangerous as this one.
"We've never encountered conditions that we're in right now," said Chandler. "It's putting all the worst-case scenarios together."
Officials have said they will raise every remaining vehicle from this watery tomb, a perilous effort that could take weeks.
"We will not be leaving victims behind," said Chandler.