  Naively, a resolution of the black hole information paradox appears to
involve microscopic details of a theory of quantum gravity. However, recent
work has argued that a unitary Page curve can be recovered by including novel
replica instantons in the gravitational path integral. Moreover, replica
instantons seem to rely on disorder averaging the microscopic theory, without a
definite connection to a single, underlying unitary quantum system. In this
letter, we show that disorder averaging and replica instantons emerge naturally
from a gravitational effective theory built out of typical microscopic states.
We relate replica instantons to a moment expansion of the simple operators
appearing in the Eigenstate Thermalization Hypothesis, describe Feynman rules
for computing the moments, and find an elegant microcanonical description of
replica instantons in terms of wormholes and Euclidean black holes.
