We go to the west coast for a different kind of water threat. They're called sneaker waves. They strike suddenly. And they can sweep away the strongest swimmer. As neal karlinsky reports, a california... See More
We go to the west coast for a different kind of water threat. They're called sneaker waves. They strike suddenly. And they can sweep away the strongest swimmer. As neal karlinsky reports, a california woman is the latest victim. Reporter: It's no surprise they're called sneaker waves. One look at the ones that have been captured on camera and you can see how suddenly they sweep in, knocking people hanging around otherwise peaceful shorelines off their feet. It's exactly what happened to 32-year-old susan archer, swept away sunday while working with her boyfriend, and their dog, trigger, along this northern california beach. You have to be careful of big waves. And we knew it. Just happened. Reporter: Tallerico said it was the same walk thek every day. Only this time, the wave came. A gun around the corner, i couldn't see them. And by the time ias getting around the corner, I saw her going out already. Reporter: This time of year from northern california to the washington coast, a simple walk on the beach can be deadly for those not keeping a close eye on the churning seas, which whip up these sneaker waves. Sneaker waves are so dangerous because they arrive on the coast as part of a smaller wave train. At first glance, the train looks like a series of normal waves. Until a much bigger and stronger sneaker wave emerges, catching everyone around unaware. Bob gibson survived one in oregon, despite being slammed by a huge log. My thought was, this cannot be happening to me. Reporter: Since november in northern california, two more sneaker waves. One killing a man on a walk with his wife. The other leaving a couple and their 16-year-old son drowned. You never, never, never turn your back on the ocean. And you never want to get really close to it. Reporter: But this morning, tallerico says people need to know. During the winter, a walk on the beach can be anything but peaceful. He'll never look at the ocean the same way again. For "good morning america," neal karlinsky, abc news, seattle.
This transcript has been automatically generated and may not be 100% accurate.