Grapnel

... is another word for a grappling hook, which is any kind of device that uses hooks to catch hold of something. In nautical terms, its suitability for use as an anchor is obvious; but it can also be used to dredge for submerged objects, or (in the days of sail) to grasp the rigging of an enemy ship to facilitate boarding.

Grappling hooks are also used by land forces, for example to set off booby traps.