  We study bubble of nothing decays and their reverse processes, the creation
of vacua through `bubbles of something', in models of the Universe based on
string theory. From the four-dimensional perspective, the corresponding
gravitational instantons contain an end-of-the-world (ETW) boundary or brane,
realized by the internal manifold shrinking to zero size. The existence of such
ETW branes is predicted by the Cobordism Conjecture. We develop the 4d EFT
description of such boundaries at three levels: First, by generalizing the
Witten bubble through an additional defect. Second, by replacing the compact
$S^1$ with a Calabi-Yau orientifold and allowing it to shrink and disappear
through a postulated defect. Third, we describe an ETW brane construction for
type IIB Calabi-Yau orientifold compactifications with O3/O7 planes through an
appropriate additional O5 orientifolding. Our 4d EFT formalism allows us to
compute the decay/creation rates for bubbles of anything depending on two
parameters: The size of the relevant defect and its tension a.k.a. the induced
(generalized) deficit angle.
