  We describe a class of wormholes that generically become traversable after
incorporating gravitational back-reaction from linear quantum fields satisfying
appropriate (periodic or anti-periodic) boundary conditions around a
non-contractible cycle, but with natural boundary conditions at infinity (i.e.,
without additional boundary interactions). The class includes both
asymptotically flat and asymptotically AdS examples. Simple asymptotically
AdS$_3$ or asymptotically AdS$_3 \times S^1$ examples with a single periodic
scalar field are then studied in detail. When the examples admit a smooth
extremal limit, our perturbative analysis indicates the back-reacted wormhole
remains traversable at later and later times as this limit is approached. This
suggests that a fully non-perturbative treatment would find a self-supporting
eternal traversable wormhole. While the general case remains to be analyzed in
detail, the likely relation of the above effect to other known instabilities of
extreme black holes may make the construction of eternal traversable wormholes
more straightforward than previously expected.
