MINNEAPOLIS (AP) — A veteran diver is slowly recovering at a Minneapolis hospital after an accident in the Michigan waters of Lake Superior led to a serious case of decompression sickness.

Terry Begnoche was shooting video of a newly discovered shipwreck off Grand Marais, Michigan, as part of his work with the Great Lakes Shipwreck Museum on Sept. 18. He had planned to make a slow ascent from 220 feet down. But a trip back up that should have taken at least 45 minutes took just two minutes, the Star Tribune reported Saturday.

Loaded with video gear, he neglected to put on a weight that would have controlled his ascent. The Whitefish Township, Michigan, man said he instantly knew he was in deep trouble, but there was no way to stop his upward momentum.

“What I was worried most about was holding my breath if I panicked,” said the 64-year-old, who has 40 years of diving experience including numerous deep-water dives. He explained that holding his breath could have caused a fatal embolism in his lungs. “Panic is what really kills people.”

After Begnoche was pulled onto the boat, he started feeling prickly sensations and lost control of his arms and legs because the nitrogen bubbles that formed in his bloodstream had cut off blood to his spinal cord and other parts of his body.

Begnoche required a helicopter rescue to Marquette, Michigan, then a low-altitude flight below 800 feet to Hennepin County Medical Center in Minneapolis where he spent 53 hours last weekend in its hyperbaric chamber, a room in which the air pressure was pumped up to give his body a chance to slowly clear out the damaging bubbles.

Doctors had to look up the proper treatment protocol in the U.S. Navy Diving Manual.

“It’s really rare to have to do that protocol for a couple reasons,” said Dr. Chris Logue, medical director of HCMC’s center for hyperbaric medicine. “One is that most people die. They never make it to treatment.”

Nurse Kimberlee Nerling, who had to stay in the chamber with Begnoche the entire time, said she noticed immediate movement in his arms when it reached its maximum pressure, equivalent to 165 feet under water.

Begnoche has since regained control of his arms. But he can’t yet walk or move his legs. He’s getting additional daily treatments in the chamber in the hopes that they will stimulate healing. Whether he can fully recover is unclear.

“It’s such a rare event, it’s impossible to tell,” Logue said. “What we’ve gained back, I’m thrilled about.”