People familiar with the Antarctic tourism industry weren't surprised that a cruise ship sank there.

What stunned them was that the ship in question was the MS Explorer, a veteran of the polar cruise ship trade, purpose-built to operate in extreme polar environments, and manned by an experienced crew. That it sank during what appears to have been the most routine of circumstances – cruising through young pack ice in mild weather – has experts scratching their heads.

"I'm totally shocked and surprised," says Leif Skog, who was captain of the Explorer for six years in the mid-1980s and early 1990s. "She was just outstanding in her design, perfect for ice navigation. It's very unlikely that pack ice caused this."

Jim Barnes, executive director of the Washington-based Antarctic and Southern Ocean Coalition, which monitors tourism and other activities, concurs. "To think [the Explorer] could sink in less than 20 hours from a relatively modest incident is very surprising," he says. "It makes you wonder if something else happened, because it really doesn't add up."

Indeed, the initial explanation of the ship's sinking on Nov. 23 – that it struck submerged ice, sprung a "fist-sized" leak, and was doomed by uncontrollable flooding – doesn't hold water for ship-design experts. Essential pieces of the story are missing, they say. Those include what the vessel really struck, why flood control efforts failed, and the timing and nature of a second collision with a large iceberg.

Doubt cast on ice-damage explanation

Sander Calisal, professor emeritus of naval architecture at the University of British Columbia, notes that Explorer's 1A-class ice-reinforced hull ought to have withstood accidental contact with submerged ice. "If there were some kind of underwater ice then, yes, there will be some impact, but I would assume it would be relatively minor." An iceberg large enough to cause serious damage would be readily visible to radar, sonar, and the eyes of the bridge crew.

Mr. Skog, Seattle-based vice president for marine operations at Lindblad Expeditions, Explorer's original owner, says collisions with submerged ice are very rare events. In a polar career spanning three decades, he can recall only a handful of times when ships he served on experienced ice damage. All amounted to dents, save one incident when a cargo ship he was commanding suffered a small, easily contained leak in the Arctic.

Further, such damage almost always occurs in the bow area, which is double-hulled as an added precaution on ice-going vessels. But the Explorer's leak had to be in the middle of the ship, he notes, because as she sank, she remained on a level, bow-to-stern trim.

The apparently small size of the puncture suggests the ship may have struck something harder than ice, according to Claude Daley, an expert in ice-reinforced ship design at Memorial University in St. John's, Newfoundland. "A fist-sized hole doesn't sound like ice damage to me," he says. "You need something very hard to cause a small hole in steel. Stone, for instance."

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