The Mk 25 Mods 2 and 3 (fig. 4-7) marine location markers are launched from aircraft or surface craft. They are launched from aircraft to provide day or night reference points for marking the course of enemy submarines in antisubmarine warfare operations. They are suitable for any type of sea-surface reference-point marking that calls for both smoke and flame for a period of 10 to 20 minutes. Mods 2 and 3 function identically. The only significant difference is that Mod 2 contains two seawater-activated batteries and two related squibs while Mod 3 contains a single battery and squib.

The Mk 25 marker consists of a cylindrical outer tube about 18.5 inches long and 2.9 inches in diameter. A valve assembly is fitted into the projecting chimney at the marker's nose end. The smoke and flame are emitted from this opening. At the opposite end is a heavier aluminum base assembly to which the outer tube is crimped. The heavy base end causes the marker to float in the water with the chimney out of the water and the base in the water. Within the base assembly is a Mk 72 Mod 0 seawater-activated battery (two batteries in the Mod 2). The battery is shielded from water contact by two plugs fitted into 1/2-inch holes on two opposite sides of the base assembly. A rigid cover (arming plate), held in place by a retainer ring, is recessed into the base end. An arrow in the center of the arming plate indicates its safe or armed position. The words SAFE and ARMED are stamped into the base rim. Also, a machined notch in the rim at the armed position helps during night use. When the arming plate is in the safe position, it physically blocks the base plugs internally to prevent them from being accidentally pushed in. When in the armed position, the arming plate no longer blocks the base plugs, allowing them to be pushed in at the appropriate time. A black rubber G-ring circles the base assembly approximately 1/4 inch from the crimp, which holds the outer case.

To activate the seawater battery, the base plugs are pushed in before the marker is actually launched. An electric squib ignites the marker, and the seawater-activated battery (two batteries and two squibs in Mod 2) supplies power. When the marker enters the water, seawater enters the battery cavity and serves as an electrolyte, causing the battery to produce a current that activates the squib. The squib ignites the starter mix, which, in turn, ignites the red phosphorous pyrotechnic composition. Gas buildup forces the valve assembly from the chimney in the nose, and yellow flame and white smoke are emitted. Burning time averages 13.5 to 18.5 minutes. Although this marker is normally used in seawater, it can be used in inland bodies of fresh water by using table salt and following the procedures outlined in Pyrotechnic, Screening, Marking, and Countermeasure Devices, NAVSEA SW050-AB-MMA-010/NAVAIR 11-15-7. 

Currently, there are two methods of launching the Mk 25 marker—by hand or by sonobuoy launcher. To launch by hand, rotate the base arming plate clockwise to the armed position. Push the base plugs into the battery cavity, located in the marker base, and throw the marker into the water. It functions as previously discussed. If the base plugs (one or both) of a marker are disturbed so the watertight integrity of the battery cavity is compromised and the marker is not launched immediately, a marine marker adapter kit (Mk 34 Mod 0) must be installed. The Mk 34 Mod 0 adapter kit is also used when the marker is launched from sonobuoy launchers. The adapter kit fits over the base end of the marker, and, when properly installed, seals the battery cavity. The adapter kit is installed by rotating the marker base arming plate to the armed position and pushing the base plugs into the battery cavity. Remove the black G-ring and install the adapter kit around the marker base so the seal plugs on the leaf-spring ends cover the holes into the battery cavity. Insert the U-pin to hold the adapter in place. To hand-launch a marker with a Mk 34 adapter kit installed, remove the U-pin and the adapter and throw the marker into the water. Before launching the Mk 25 marker from sonobuoy launchers, you should install a Mk 34 adapter kit (fig. 4-8) on the marker. The adapter kit provides a safe environment for the marker until the last possible moment before launch. Because of the physical difference in size between the Mk 25 marker and the sonobuoy launch tubes, a sonobuoy launch container (SLC) and spacer assembly (fig. 4-8) is used to launch the Mk 25 marker from a sonobuoy launcher. The Mk 25 marker, with the Mk 34 adapter kit installed, is installed into the cavity half of the split foam spacer assembly section with the U-pin of the Mk 34 adapter in the UP position. Then, remove the U-pin and place the other half of the split foam spacer assembly section over the marker, which completely encloses the marker. The foam spacer assembly containing the marker is then installed into the SLC against the obturator. Foam pads are installed, and the end cap is locked into slots in the SLC, retaining the assembly inside the SLC. A JAU-1/B cartridge is installed in the SLC, and the loaded SLC is installed in the sonobuoy launcher aboard the aircraft. To launch the Mk 25 marker, the JAU-1/B cartridge is initiated through the aircraft circuitry. It develops sufficient gas pressure inside the SLC to force off the SLC end cap and expel the foam spacer assembly containing the Mk 25 marker. Once out of the SLC, the split foam spacer assembly is separated by the airstream, freeing the marker. The airstream then separates the Mk 34 adapter kit from the marker, allowing them to fall to the water as separate units. Upon entry into the water, the marker functions as previously discussed.