"", released 2001 for the PlayStation 2, further evolved the stealth gameplay series. It featured an array of new abilities, including "leaping over and hanging off of railings, opening and hiding in storage lockers," and sneaking up behind enemies to "hold them at gunpoint for items and ammunition." "Metal Gear Solid 2" holds a Guinness World Record for being the first stealth game to feature collective artificial intelligence. "Metal Gear Solid 2: Sons of Liberty" sold 7 million units in sales, followed by "Metal Gear Solid" with 6 million units.