MCRP 12-10B.1, page 300

c. Marines entering the sewer should carry a sketch of the sewer system that includes magnetic north, azimuths, distances, and manholes. They should also carry protective masks, flashlights, gloves, and chalk (or chemiluminescent lights) for marking features along the route. The patrol should also be equipped with a safety rope to which each Marine is tied. To improve their footing in slippery sewers and storm drains, the squad members may wrap chicken wire or screen wire around their boots. d. A constant concern to units conducting a subterranean patrol is chemical defense. Enemy chemical agents used in tunnels may be suddenly encountered in dense concentrations with little dispersion. The point person should carry an automatic chemical agent alarm system to provide instant warning of the presence of chemical agents. M8 detector paper can also be used to detect chemical agents. At the first indication that harmful gases are present, the squad should mask. e. In addition to enemy chemical agents, noxious gases from decomposing sewage can also pose a threat. These gases are not detected by chemical agent alarm systems, nor are they completely filtered by the protective mask. Physical signs that indicate their presence in harmful quantities are nausea and dizziness. The squad leader should be constantly alert to these signs and know the shortest route to the surface for fresh air. Some sewer gases deplete oxygen, a situation against which the field protective mask provides no protection. f. Once the squad is organized and equipped, it moves to the entrance of the tunnel. This may be a maintenance hole. With the maintenance hole cover removed, the squad waits 15 minutes before entry to allow any gases that may be present to dissipate. The point person then descends into the tunnel to determine whether the air is safe to breathe and whether movement is restricted. The point person should remain in the tunnel for 10 minutes before the rest of the squad follows. If he/she becomes ill or is exposed to danger, he/she can be pulled out by the safety rope. g. When the squad is moving through the tunnel, the point person moves about 10 meters in front of the squad leader. Other squad members maintain 5-meter intervals. If water in the tunnel is flowing faster than 2.5 meters per second or the sewer contains slippery obstacles, those intervals should be increased to prevent all patrol members from falling if one Marine slips. All squad members should stay tied into the safety rope so that they can easily be retrieved from danger. The last Marine in the patrol marks the route with chalk or chemiluminescent lights so other units can find the squad. h. The squad leader should record the azimuth and pace count of each turn he/she takes in the tunnel. When he/she encounters a maintenance hole to the surface, the point person should open it and determine the location, which the patrol leader then records. The use of recognition signals (Figure E-4) prevents friendly forces from accidentally shooting the point person as he/she appears at a maintenance hole. E - 4 MCWP 3-35 .3