Undersea tremors or other environmental conditions can cause a sudden rising of the sea followed by a rapid, powerful withdrawal. These surges are dangerous and hard to predict, though they are most likely to occur in the far tidelands, nearest the water.

Because of their unpredictable and deadly nature, these waves are a source of superstition for the tidal nomads. Legend says that a mourning wave is the sea claiming something it is owed, and those who believe these legends often preemptively leave items behind to be caught by the tiderise. Some leave broken or useless things, believing it can be tricked; others say it will know if the gift is unworthy and will become even more vengeful.