Living in underground tunnels presents mammals with unique communication challenges. The vocal repertoire of subterranean rodents may be extensive, especially in social species, but airborne sound propagates poorly underground and vocalizations are constrained to the low-frequency range. The hearing ranges of subterranean mammals are also limited to low frequencies, but audition does not appear to be unusually sensitive in these animals. Seismic signaling may be required for communication over long distances, such as between burrows. Although well-documented in spalacid and bathyergid mole-rats, the mechanisms of perception of the low-frequency vibrations that they generate remain controversial. Less is known about hearing and communication in non-rodent subterranean species, but clues are available from middle ear morphology. For example, some golden moles (Chrysochloridae) have enormously enlarged middle ear ossicles, which may be used for seismic navigation and prey detection. The morphology and behavior of subterranean mammals can be bizarre and enigmatic, but generalities are emerging, and their bioacoustic world is being progressively uncovered.