  We explore the mean and fluctuating redshifted 21 cm signal in numerical
simulations of cosmic reionization from the Cosmic Reionization On Computers
(CROC) project. We find that the mean signal varies between about
$\pm20\rm{mK}$. Most significantly, we find that the negative pre-reionization
dip at $z\sim10-15$ only extends to $\langle\Delta T_B\rangle\sim-20\rm{mK}$,
in agreement with prior simulation results and in significant contrast to
Pritchard & Loeb analytical model, requiring substantially higher sensitivity
from global signal experiments that operate in this redshift range (EDGES-II,
LEDA, SCI-HI, and DARE). We also explore the role of dense substructure
(filaments and embedded galaxies) in the formation of 21 cm power spectrum. We
find that by neglecting the semi-neutral substructure inside ionized bubbles,
the power spectrum can be mis-estimated by 25-50\% at scales $k\sim
0.1-1h\rm{Mpc}^{-1}$. This scale range is of a particular interest, because the
upcoming 21 cm experiments (MWA, PAPER, HERA) are expected to be most sensitive
within it.
