  Before the universe was reionized, it was likely that the spin temperature of
intergalactic hydrogen was decoupled from the CMB by UV radiation from the
first stars through the Wouthuysen-Field effect. If the IGM had not yet been
heated above the CMB temperature by that time, then the gas would appear in
absorption relative to the CMB. Large, rare sources of X-rays could inject
sufficient heat into the neutral IGM, so that the differential brightness
temperature was greater than zero at comoving distances of tens to hundreds of
Mpc, resulting in large 21-cm fluctuations with amplitudes of about 250 mK on
arcminute to degree angular scales, an order of magnitude larger in amplitude
than that caused by ionized bubbles during reionization, about 25 mK. This
signal could therefore be easier to detect and probe higher redshifts than that
due to patchy reionization. For the case in which the first objects to heat the
IGM are QSOs hosting 10^7-solar mass black holes with an abundance exceeding
about 1 per Gpc^3 at z~15, observations with either the Arecibo Observatory or
the Five Hundred Meter Aperture Spherical Telescope (FAST) could detect and
image their fluctuations at greater than 5-sigma significance in about a month
of dedicated survey time. Additionally, existing facilities such as MWA and
LOFAR could detect the statistical fluctuations arising from a population of
10^5-solar mass black holes with an abundance of about 10^4 per Gpc^3 at
z~10-12.
