The Leviathan, based on the UH-3 Sea King, appears Grand Theft Auto: San Andreas. It is the second largest helicopter on the game, and has the ability to land on water thanks to its boat-shaped hull. It is very slow and sluggish, and strangely has the same rotor sound as the Hunter, Cargobob and Raindance.
It has been used in the mission Up, Up and Away with a crane with a magnet attached to the bottom, that can lift vehicles. There is NO way to obtain a regular helicopter with a magnet like this outside of this mission. The only way to obtain it is that you must take the Leviathan (with the magnet in it, and while on the mission) into the Verdant Meadows airstrip garage (the one that stores aircrafts) to store your helicopter. Once you have stored your Leviathan with the magnet inside the garage, blow it up to fail the mission, get out of the garage and wait for the large doors to close. After the door closes, enter the garage once more. there will be a Leviathan with a magnet waiting for you inside, the next time you come and visit.
Once completing that mission a Leviathan will spawn at Verdant Meadows; your private airstrip, but it doesn't come with a magnet to grab vehicles anymore.