  We argue that expanding bubbles of nothing are a widespread feature of
systems of black holes with multiple or non-spherical horizons, appearing as a
limit of regions that are narrowly enclosed by the horizons. The bubble is a
minimal cycle that links the Einstein-Rosen bridges in the system, and its
expansion occurs through the familiar stretching of space in black hole
interiors. We demonstrate this idea (which does not involve any Wick rotations)
with explicit constructions in four and five dimensions. The geometries of
expanding bubbles in these dimensions arise as a limit of, respectively, static
black hole binaries and black rings. The limit is such that the separation
between the two black holes, or the inner hole of the black ring, becomes very
small, and the horizons of the black holes correspond to acceleration horizons
of the bubbles. We also explain how a five-dimensional black hole binary gives
rise to a different type of expanding bubble. We then show that bubble
spacetimes can host black hole binaries and black rings in static equilibrium,
with their gravitational attraction being balanced against the background
spacetime expansion. Similar constructions are expected in six or more
dimensions, but most of these solutions can be obtained only numerically.
Finally, we argue that the Nariai solution can be regarded as containing an
expanding circular bubble of nothing.
