The Claw of Archimedes (Ancient Greek: Ἁρπάγη, romanised: harpágē, lit. ’snatcher’; also known as the “iron hand”) was an ancient weapon devised by Archimedes to defend the seaward portion of Syracuse’s city wall against amphibious assault.
Although its exact nature is unclear, the accounts of ancient historians describe it as a sort of crane equipped with a grappling hook that was able to lift an attacking ship partly out of the water, then either cause the ship to capsize or suddenly drop it. It was dropped onto enemy ships, which would then swing itself and destroy the ship.
These machines featured prominently during the Second Punic War in 214 BC, when the Roman Republic attacked Syracuse with a fleet of sixty quinqueremes under Marcus Claudius Marcellus. When the Roman fleet approached the city walls under cover of darkness, the machines were deployed, sinking many ships and throwing the attack into confusion. Historians such as Livy attributed heavy Roman losses to these machines, together with catapults also devised by Archimedes.
The plausibility of this invention was tested in 1999 in the BBC series Secrets of the Ancients and again in 2004 in the Discovery Channel series Superweapons of the Ancient World. The producers of Superweapons brought together a group of engineers tasked with conceiving and implementing a design that was realistic, given what is known about Archimedes. Within seven days they were able to test their creation and succeed in tipping over a model Roman ship
You can find scale models of the claw here.