  Despite immense advances in the field of topological materials, the
antiferromagnetic topological insulator (AFMTI) state, predicted in 2010, has
been resisting experimental observation up to now. Here, using density
functional theory and Monte Carlo method we predict and by means of structural,
transport, magnetic, and angle-resolved photoemission spectroscopy measurements
confirm for the first time realization of the AFMTI phase, that is hosted by
the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering
makes MnBi$_2$Te$_4$ invariant with respect to the combination of the
time-reversal ($\Theta$) and primitive-lattice translation ($T_{1/2}$)
symmetries, $S=\Theta T_{1/2}$, which gives rise to the $Z_2$ topological
classification of AFM insulators, $Z_2$ being equal to 1 for this material. The
$S$-breaking (0001) surface of MnBi$_2$Te$_4$ features a giant bandgap in the
topological surface state thus representing an ideal platform for the
observation of such long-sought phenomena as the quantized magnetoelectric
coupling and intrinsic axion insulator state.
