Sneaker waves are disproportionately large ocean waves which appear suddenly and without any warning at all. Since they are unexpected, they represent a serious hazard to both people and ship traffic. Some regions of the world are more prone to these waves than others, leading to posted warnings and advisories to make visitors aware of the issue. Any ocean or large body of water has the potential to form sneaker waves, however, so people should always be cautious around water.
Unlike a tsunami, a sneaker wave is not a predictable event. These waves form when several smaller waves combine their energy, creating a single big wave. This can happen as a result of the disruption of ocean currents, or because of subtle changes in weather and topography. They follow no known pattern, and they will interrupt the established wave pattern, so even if people think they know the water, they should be careful. They are very strong, and they can kill.
In most places around the world, waves flow more or less in cycles. These cycles can be studied and predicted; this is a skill which is familiar to many surfers, fishers, and other people who spend a lot of time around the ocean. Once people learn wave patterns, they can be aware of predicted lulls and larger waves. Knowing the predominant wave pattern in an area will not protect someone from sneaker waves, however.
The height of a sneaker wave can be significantly higher than the waves which precede and follow it. These waves have been known to crest headlands, pulling down hikers and ocean-watchers. They can certainly yank someone from a beach or rock piling, and they can also carry debris which may cause serious injuries. On the open ocean, sneaker waves have swamped ships; even massive tanker ships are not immune to the dangers of freak waves.
When anyone is around a large body of water, he should protect himself from sneaker waves by never turning his back to the water. If someone wants to nap on the beach, she should make sure that a companion stays awake to keep an eye on things. Small children and pets should be watched, as they may not be aware of the danger, and everyone should avoid standing close to the edge of promontories and headlands. It is important to make sure there is a clear escape route away from the ocean, and people should avoid slippery jetties and docks.