FALCON COVE — Arch Cape residents and visitors reported a large whale stranded on a sandbank offshore about 250 yards out Friday.

As night fell, the humpback was driven by waves southward toward Falcon Cove. For more than two hours, the bloated carcass drifted closer to shore as the tide shifted. What had once been a blur on the horizon grew larger and larger. A handful of onlookers came out to view the beaching. They had recently witnessed a Japanese fishing boat from the tsunami wash ashore, but none remembered a whale. Holding flashlights and cellphones, wading through tidepools, they tried to capture the image.

Leslie Smith was watching from a bluff before coming down to Cove Beach. “We could see it way out,” she said. “There were extremely long flippers.”

“We’ve had baby orcas, but never a big bull like this,” Smith added, as her daughter Lindsay Smith snapped a photo.

Cape Falcon’s John Erben said, “We see a lot of whales. Most times they’re not floating dead.”

On Saturday, Seaside Aquarium General Manager Keith Chandler visited the site. He said he had been notified of the whale by the U.S. Coast Guard Friday morning, when the whale was about 4 miles offshore.

“It’s been dead quite some time,” Chandler said. “Its tail has been rotted off.”

The big bloat of the whale is gas built up inside, Chandler said.

Does it pose a risk? Could it explode?

“It could,” Chandler said. “They have in the past. Not saying it will, but it’s always a possibility.”

Humpbacks are more uncommon than gray whales, Chandler said. “We get one every couple of years. We had one in Seaside last January. There have been some live ones in the Columbia too.” As for what caused the whale’s death, “There was no smoking gun,” he said.

Chandler said the tide could move the whale carcass out of the cove and farther south.

“If we can get to it, we’ll probably do a necropsy,” he said. “But one that has been dead this long — I don’t know how much we’re going to learn from it, because all the tissues are probably rotting away.”

By late Saturday, the humpback remained ashore in Falcon Cove, but neighbors reported the whale had deflated significantly.