  We report broadband microwave absorption spectroscopy of the layered
antiferromagnet CrCl3. We observe a rich structure of resonances arising from
quasi-two-dimensional antiferromagnetic dynamics. Due to the weak interlayer
magnetic coupling in this material, we are able to observe both optical and
acoustic branches of antiferromagnetic resonance in the GHz frequency range and
a symmetry-protected crossing between them. By breaking rotational symmetry, we
further show that strong magnon-magnon coupling with large tunable gaps can be
induced between the two resonant modes.
