During the Vietnam War, the USAF developed a whole series of downward ejecting dispenser pods that were authorized for use by the F-105 Thunderchief, the F-111, and the F-4 Phantom, but in practice only the Phantom used the pods. The pods varied little in external appearance and remained with the aircraft but the submunition payloads were a diverse range of air-dropped area denial mines designed for use against vehicles and enemy troops.
The most interesting of the payloads combined the SUU-41 dispenser pod with what was called the "Gravel" mine. The Gravel mine was a small antipersonnel mine triggered by the pressure of someone stepping on it- though it didn't kill, it certainly caused ruined the foot. The mine was essentially an explosive laden bag of cloth with no type of fusing as the explosive was simply triggered by pressure. To keep the mine inert while in the SUU-41 dispenser, they were packed in liquid Freon and as such, could only be carried and dropped by specially-modified F-4Ds with monitoring equipment to use the Gravel mine.
The WSO in the backseat monitored the temperature of the ten payload bays in each of the SUU-41 pods. Once the Gravel mines warmed to a certain temperature, warning lights came on to alert the flight crew that the mines were "live" in five minutes. Ideally, the mines were dropped while still cold and as the Freon evaporated, the mines would go live. After a period of time, mine's explosives went inert.
Only a handful of Phantoms assigned to the 8th Tactical Fighter Wing at RTAFB Ubon in Thailand were able to use the Gravel mines.
Source: McDonnel F-4 Phantom- Spirit in the Skies, Jon Lake and David Donald, eds. AIRtime Publishing, 2002, p136.