LunarMax is a prison built by MIB located on Earth's Moon. It was built at some point after the Moon Landing in 1969 and is designed to hold alien prisoners, and where Boris the Animal is imprisoned by Agent K for the murder of Colonel James Darrell Edwards II and for attempting to stop the deployment of the Arc Net Shield which would have allowed his race to invade the Earth.
Boris was chained up like an animal while he spent the next forty years but he broke out of his cell with the assistance from Lily the Poison, who sneaked Weasel inside a cake. With the help of Weasel, Boris easily took down all the guards that got on their path and reached the armory where he and Lily took a powerful laser weapon to ease the escape.
Boris killed Obadiah Price before he and Lily faced off with the prison's security. Boris decided to "open a window" by using the weapon he stole to blast a hole above them, causing a violent decompression that blew the security and Lily outside the prison and sent them into the vacuum of space, while Boris stood attached to the floor avoiding the same fate thanks to his clawed feet, before jumping back in 1969 to alter history by killing K and preventing the Arc Net's launch, allowing the Boglodites to invade the Earth in 2012.
Once Agent J fixed the timeline, Boris was killed by K instead of being arrested, thus his escape from LunarMax was erased from history and everyone he murdered in 2012 is alive.