  We study CMB secondary anisotropies produced by inhomogeneous reionization by
means of cosmological simulations coupled with the radiative transfer code
CRASH. The reionization history is consistent with the WMAP Thomson optical
depth determination. We find that the signal arising from this process
dominates over the primary CMB component for l > 4000 and reaches a maximum
amplitude of l(l+1)C_l/2\pi ~ 1.6 x 10^{-13} on arcmin scale, i.e. l as large
as several thousands. We then cross-correlate secondary CMB anisotropy maps
with neutral hydrogen 21cm line emission fluctuations obtained from the same
simulations. The two signals are highly anti-correlated on angular scales
corresponding to the typical size of HII regions (including overlapping) at the
21cm map redshift. We show how the CMB/21cm cross-correlation can be used to:
(a) study the nature of the reionization sources, (b) reconstruct the cosmic
reionization history, (c) infer the mean cosmic ionization level at any
redshift. We discuss the feasibility of the proposed experiment with
forthcoming facilities.
