  The fluctuations in the emission of redshifted 21cm photons from neutral
inter-galactic hydrogen will provide an unprecedented probe of the reionization
era. Conventional wisdom assumes that this 21cm signal disappears as soon as
reionization is complete, when little atomic hydrogen is left through most of
the volume of the inter-galactic medium (IGM). However observations of damped
Ly-alpha absorbers indicate that the fraction of hydrogen in its neutral form
is significant by mass at all redshifts. Here we use a physically-motivated
model to show that residual neutral gas, confined to dense regions in the IGM
with a high recombination rate, will generate a significant post-reionization
21cm signal. We show that the power-spectrum of fluctuations in this signal
will be detectable by the first generation of low-frequency observatories at a
signal-to-noise that is comparable to that achievable in observations of the
reionization era. The statistics of 21cm fluctuations will therefore probe not
only the pre-reionization IGM, but rather the entire process of HII region
overlap, as well as the appearance of the diffuse ionized IGM.
