  We consider entanglement entropies of finite spatial intervals in Minkowski
radiation baths coupled to the eternal black hole in JT gravity, and the
related problem involving free fermion BCFT in the thermofield double state. We
show that the non-monotonic entropy evolution in the black hole problem
precisely matches that of the free fermion theory in a high temperature limit,
and the results have the form expected for CFTs with quasiparticle description.
Both exhibit rich behaviour that involves at intermediate times, an entropy
saddle with an island in the former case, and in the latter a special class of
disconnected OPE channels. The quantum extremal surfaces start inside the
horizon, but can emerge from and plunge back inside as time evolves,
accompanied by a characteristic dip in the entropy also seen in the free
fermion BCFT. Finally an entropy equilibrium is reached with a no-island
saddle.
