For three years, Pennsylvania-born scuba diver Chris Donnelly prepared for the underwater expedition of a lifetime: a journey to the SS Andrea Doria . The so-called “Mount Everest of Shipwrecks,” it’s located off the coast of Massachusetts at a depth of 250 feet. Donnelly, a technical diving instructor trainer who operates the Philadelphia-based Blue Crew Divers center, had been exploring shipwrecks since he was 16, racking up thousands of certification hours at dive sites around the world, but the Andrea Doria posed a unique challenge.

“It’s one of the coldest, darkest wrecks in the world,” he says. “There’s a lot of drama and ego surrounding it.” Despite careful preparations, not everything went according to plan during his first mission to the famed wreck, a 700-foot Italian luxury liner that sank in 1956 after colliding with another vessel, killing 46 passengers.

(The wreck has also claimed the lives of 18 scuba divers.) Rough seas kept the divers out of the water for two days, and when the conditions finally improved and Donnelly took a long stride off the boat and into the water, he was wearing so much weighted gear that one of his fins snapped upon contact with the water’s surface. “I remember thinking, ‘Am I good enough to do this? Have I done enough training?’” he recalls.

After regaining composure, Donnelly began his descent, feeling for the anchor line as he sank lower and lower. At around 180 feet, he caught his first glimpse of the gho.