Quicksand is a block from ExtrabiomesXL. It can be found in relatively deep pockets in mini jungles. Standing on a quicksand block will cause you to sink, resulting in block suffocation unless the quicksand block is broken. It can be collected by hand, but using a shovel is much quicker.
Quicksand can be used for a mob trap or mob grinder, as seen below.
This mob trap that uses Quicksand. When a mob, lured by the player, walks over the quicksand, they slowly sink into it and suffocate. Upon death, their drops enter the room underneath the quicksand, allowing easy collection of resources such as gunpowder, string, rotten flesh, and more.