  We consider a primordial black hole of very high mass, $10^9-10^{10}M_\odot$,
surrounded by dark matter and barionic halo at redshifts $z\sim20$ without any
local sources of energy release. Such heavy and concentrated objects in the
early universe were previously called ``cosmological dinosaurs''. Spectral
distribution and spatial variation of the brightness in the 21 cm line of
atomic hydrogen are calculated with the theory of radiation transfer. It is
shown that a narrow and deep absorption arises in the form of the spherical
shell around the primordial black hole at the certain radius. The parameters of
this shell depend almost exclusively on the mass of the black hole. The
hardware and methodological aspects of the search for such objects are
discussed.
