The events known as incursions are the result of a contraction in the Multiverse's timeline. They exist in a paradoxical nature, as this contraction was caused by the early death of an unknown universe approximately 25 years in the past, when its Molecule Man was killed by the Molecule Man of Earth-616. However, said Molecule Man killed his counterpart when he was investigating the incursions with Doctor Doom.
This contraction caused universes throughout the Multiverse to collide, with each universe's respective Earth as the point of impact. The incursion is the name given to the period of time in which the two Earths collide, and lasts exactly eight hours. The first incursion happened seven years after the first Molecule Man was killed. Throughout the duration of an incursion, there is a short period of harmonic alignment which allows both Earths to exist next to each other without any detrimental effects.
Once the eight hours pass and the Earths collide, both the planets and their respective universes are obliterated. According to the Black Swan, there are at least eight ways of averting an incursion. One of these involves the destruction of at least one of the Earths before they can collide. Doing so will save both universes, and one of the Earths. Each time a new incursion happens, the rate in which universes collide accelerates. With each universe that is destroyed, the Multiverse contracts and its lifespan is shortened.
For each incursion there is an "incursion point," a single area in each of the colliding Earths from which the other Earth can be seen approaching, the sky inside the incursion point can be of varying colors, while usually colored red, it can also be blue which portends the Mapmakers. Outside the incursion point, everything looks normal. An unusual phenomenon called "Simtum Tapputu" can occur, in which an Earth is in incursion with two realities at the same time.