Longship
Longships for the most part used two different kinds of anchors. The most common was a natural wood yoke formed from a tree branch. The weight was supplied by a stone passing laterally through the U of the yoke. The top of the yoke was closed by either a length of hardwood or a curved iron head, which kept the stone in place. One side of the head stuck out so it could dig into mud or sand. In the Ladby ship burial in Denmark, a unique iron anchor has been found, resembling the modern fisherman's anchor but without the crossbar. The cross bar may have rusted away. This anchor—made of Norwegian iron—has a long iron chain to which the hemp warp was attached. This construction has several advantages when anchored in deep waters or in rough seas.
Vacuum-anchor
In large scale oceanic civil engineering, vacuum-anchors are ocean bottom fasteners used to anchor deep seas oil platforms (example: See Troll A Oil Platform) and other gravity base technology based structures in the soupy soft bottomed muck found on many oil bearing continental shelves and the world's shallower seas. Gravity based anchor legs flare out forming a wider footprint where they contact the bottom much as the webbed feet of many aquatic animals increase the surface area above the vacuum-anchors which are open ended long cylindrical shaped deep cups with a sealed top end closed by a valve which connects and anchors mechanically to the legs of the gravity based structure. The valve is used to exhaust any gases and liquid which seeks to escape the trapped sea bottom materials entering the open end of the anchor.