Underspatial boundary

The boundary that separates the four dimensions of our space-time continuum from the dimensions of Underspace, also referred to as the "Portal Dimension". Ordinary matter common to our Universe cannot pass through this boundary in any shape or form naturally; the only way to make the transit is to be confined in an artificial interdimensional stasis field, such as the one produced by the PlasmaBall Dimensional PropSys or a stationary Gateway Device.