The Sonic Resonator was a device created by the Coalition of Ordered Governments to map underground tunnels. The Sonic Resonator works on the active (as opposed to passive) sonar principle, by emitting powerful pulses of sound, then receiving the waves as they bounce back off the rock of the Hollow. Based on the distortion, strength and other properties of the received pulses, it can generate a 3-D map of the surrounding tunnel systems. This data was presumably then transmitted by the Resonator back to Control. It is possible that the resonator employs explosives to generate these sound waves, as Marcus Fenix refers to the device as having been 'detonated', although it may be some other unexplained technology. In any case, the sound pulses are extremely powerful, having the ability to shatter solid stone hundreds of metres away in any direction. The Sonic Resonator was carried by Alpha Team to map out the Locust tunnels, and provide targeting data for the deployment of the Lightmass Bomb, but they were wiped out and Delta Team had to transport and deploy the Resonator instead. The Resonator was successfully deployed at the Lethia Imulsion Facility, but much to the COG's outrage, the device failed to operate properly and only provided intelligence on an insignificant fraction of the Hollow. Despite this failure, Delta Squad was able to locate a much more complete source of data on the Hollow from the laboratory of the late scientist Adam Fenix. How Adam Fenix was able to map such an extensive amount of the Hollow without Resonator technology is unexplained, however he almost certainly did not use Resonator technology due to its ineffectiveness, even years later.
The Resonator is carried throughout the Gears of War campaign by Augustus Cole after Delta Squad recovered the remnants of Alpha Squad. The Sonic Resonator, however, proved to be an ineffective way of mapping tunnels when COG Control reported that it had only mapped a fragment of the necessary area due to insufficient data on the Locust network. The Resonator was left at the imulsion facility and was likely destroyed when the Hollow was eventually destroyed by the Lightmass Bomb.