Quicksand.

Quicksand is a loose, light sand mixed with water. It looks no different from ordinary sand, but it won't support any weight. 
Quicksand can be usually found near the mouths of large rivers or on flat shores where there is a stiff layer of clay below ground level. 
This clay stops the natural drainage of water, so the water collects in the sand. It's a common belief that quicksand will suck you down but, in fact, it is possible to float, as long as you move slowly. 
Moving slowly enables the sand to flow around tha body and acts like water in which you can swim. 

(Thanks to Lorenzo S. for helping me writing this post)