"It was absolutely insane, a massive experience. I'm sort of humble and filled with awe," Frode Nilsen told TV2.
The two men drove out to the site on Monday, a day before the whale was herded back to the open sea, and slipped into the water wearing wetsuits, flippers and goggles.
One of the divers, Anders Ropeid, 36, had past experience of swimming with whales, and knew how to avoid alarming the animal by diving with oxygen tank and being extremely calm.
"We took it very quietly and made little sound," says Nilsen, 32. "He came close to us several times, but as he approached he slowed down."
The two divers swum around the whale for two hours. At one point there were only a few metres separating them from the massive beast.