A couple on a whale-watching trip off Cape Town, South Africa, say they had a lucky escape when a 10m (33ft) specimen leapt on to their yacht.
The southern right whale, a species known for poor eyesight, snapped the mast before sliding back into the water, said Paloma Werner.
She and her partner had just seconds to take cover, she said. A nearby tourist caught the moment on camera.
"I still like whales," Ms Werner told the BBC afterwards.
The southern right whale navigates by sound, leading Ms Werner to suspect it was an accident.
"Our boat's engine was off and so the whale just didn't know we were there," she said.
"We were just the wrong boat, in the wrong place at the wrong time."
Diving for cover
Ms Werner and her companion Ralph Mothes had headed out to Table Bay on their yacht in fine weather before catching sight of the whale in the distance.
Cutting the engine, they floated for an hour watching the whale, she told the BBC.
"It appeared about 120m away from our boat and then it went under the water again.
"A few moments later, I saw it resurface just 10m away.
"Suddenly I heard my partner shout and when I looked around, I saw the huge thing breaching on to the deck.
"Instinctively, I took cover as the mast came crashing down. I saw my partner, Ralph, dive for cover behind the yacht's wheel. Then the whale slid down the side of the boat and back into the water."
"As soon as I realised that we were unhurt, we checked to see how much damage had been done.
"Luckily, we weren't taking on water so we started the engine and headed for shore. I saw the whale reappear further away.
"When we got back to land, I realised how lucky we were to have survived."
The leaping whale was caught on camera, apparently by a tourist on another boat who later passed it on through a tour guide.