Freeze

The Freeze utility will seal all the worms on your team inside an ice cube. Frozen worms cannot be damaged, with a few exceptions. This utility is useful if your worms are low on health, to make your opponents target other team. The worms can still be moved by explosions, so be careful about drowning.

If a player drops a Petrol Bomb on a frozen worm, the worm will take damage from the fire the moment it thaws out. The same thing happens with a Mine, as soon as it thaws, the countdown of the mine begins.

Wormopedia Entry
The mighty Freeze.

''It's great to use when one is about to be involved in a sudden death battle. It provides a nifty shield from the energy-sapping nuke which means you won't get infected, great huh? It also protects you from all other forms of damage. Be careful not to use the Freeze when you haven't got much land beneath you, a well placed dynamite could see you sinking into the murky depths!''

Trivia

 * Even though it is not a "true" weapon, Freeze is counted as a super weapon.