Separating the Earth from the Umbrae is a metaphysical "membrane" known as the Gauntlet. Some humans almost manage to get close to the Gauntlet in times of bliss, meditation, or creativity; this state is known as the Periphery.
The Gauntlet is thinner in places such as a haunted graveyard or the open wilderness, and thicker in places such as laboratories and shopping malls. The strength of the Gauntlet in a given area determines the ease with which one can cross it, or "step sideways." The Gauntlet is usually treated as a single phenomenon, when it is actually sort of three Gauntlets in one: one separating the Earth from the Astral Plane, one protecting it from the Spirit Wilds, and one blocking access to the Underworld and the horrors that lie within. The latter Gauntlet is also referred to as the Shroud by wraiths, and as the Stormwall by agents of the Orpheus Group.
In 1999 CE, the Avatar Storm has raged within the Gauntlet, composed of jagged shards of souls and Avatars. It does not affect most supernaturals, but the shards are drawn to the Avatars of mages. The more powerful the Avatar (and the higher the mage's enlightenment), the more the shards are drawn to him, and thus the more damage is potentially done when stepping sideways.
Though by 2015 CE the Gauntlet has returned to its former self so once again mages can pass through the Gauntlet as before though the Umbra beyond is different to the Umbra before the Avatar Storm.