The state ferry dock in Skagway, Alaska remains underwater and is closed indefinitely after it sank on early Thursday morning.

"Nobody's really sure what happened... but it was a surprise," says Skagway resident Gary Heger, who was one of the first to see the dock submerged on Thursday morning.

By about 9:30 a.m., the dock was fully submerged and the passenger walkway had collapsed.

The dock, about 40 by 50 metres in size and made up of 24 concrete chambers, is normally used to load passengers and vehicles onto Alaska Marine Highway System ferries.

It's now fully submerged at high tide and partially submerged during low tide.

State transportation engineers and a diver have been on site and are still assessing the damage and what could have caused the dock to give way.

"The word that the engineer used is he's perplexed," says Jeremy Woodrow, a spokesman with the Alaska Department of Transportation, who says the dock is resting on the seafloor.

"We actually have inspected every one of those individual chambers [in the dock] in the last year and there was no indication of any wear."

Yesterday, yellow absorbent boom was laid around the structure.

The Alaska Marine Highway System has cancelled all sailings for the next two weeks.

Woodrow says if the dock can be salvaged, it will take some time before it will be ready to accept ferry traffic.

"We're working with the municipality of Skagway and other marine facilities to see if there's a temporary solution that we can put in place with our ferry system," he says, "or maybe we need to bring in a temporary floating dock so we can restore ferry service."