One cannot normally alter the Marvel Universe's history; if a time-traveler should cause an alteration to the established flow of events at some point in the past, a divergent universe will simply "branch out" from the existing timeline, and the time-traveler will still return to his or her unaltered original universe. Those realities can also spawn realities of their own. There exist hundreds, probably thousands of such realities. It is unknown why this happens, though a warp known as the Nexus of All Realities exists in a swamp in the Florida Everglades of Earth-616. For the most part, this does not matter, as most beings are unaware that this occurs, or even that their universes were recently "born" from another. However, individuals and organizations exist that try to monitor or manipulate the various realities. These include Immortus, the Captain Britain Corps, the Time Variance Authority, the Timebreakers/Exiles, and Kang the Conqueror's forces. It is possible to travel through time without creating a new alternative universe, instead of altering events in the future, but this seems to have devastating and very far-reaching repercussions, as depicted in Marvel 1602 (it almost destroyed the whole multiverse, including the afterlife).