  We study the motion of thin-shell bubbles and their tunneling in anti de
Sitter (AdS) background. We are interested in the case when the outside of a
shell is a Schwarzschild-AdS space (false vacuum) and the inside of it is an
AdS space with a lower vacuum energy (true vacuum). If a collapsing true vacuum
bubble is created, classically it will form a Schwarzschild-AdS black hole.
However, this collapsing bubble can tunnel to a bouncing bubble that moves out
to spatial infinity. Then, although the classical causal structure of a
collapsing true vacuum bubble has the singularity and the event horizon,
quantum mechanically the wavefunction has support for a history without any
singularity nor event horizon which is mediated by the non-perturbative,
quantum tunneling effect. This may be regarded an explicit example that shows
the unitarity of an asymptotic observer in AdS, while a classical observer who
only follows the most probable history effectively lose information due to the
formation of an event horizon.
