Quicksand

Morrish Bible Dictionary

ἡ Σύρτισ. The Syrtis is a quicksand on the north African coast between long. 10 and 20 E.  Acts 27:17 . There are properly two, called the major and the minor; it would have been the major one on the east of the bay that they were in danger of. It is now called the Gulf of Sidra.

Webster's Dictionary

(n.) Sand easily moved or readily yielding to pressure; especially, a deep mass of loose or moving sand mixed with water, sometimes found at the mouth of a river or along some coasts, and very dangerous, from the difficulty of extricating a person who begins sinking into it.

The Nuttall Encyclopedia

Sandbank so saturated with water that it gives way under pressure; found near the mouths of rivers.