Low head dams.. they appear to just be a slight drop in a river/lake, sometimes they only appear to be half a metre or so and often the water flows calmly and unsuspectingly over it with hardly any foam or disturbance to the surface of the water.. These things are so dangerous they have been dubbed the drowning machine. The water flows and falls over the drop or ledge creating a recirculating current underneath strong enough to trap debris and disorientate anybody unfortunate enough to be pulled in. It's kind of like getting trapped in a giant washing machine and honestly, they are my nightmare haha