text: Offensive Considerations in Counterinsurgency
  Subterranean Area Searches
  5-123. In both urban and rural areas, insurgents must remain undetected to survive. Therefore, insurgents
  use all means of concealment available. Subterranean areas, in both rural and urban areas, reduce the
  chance of detection and facilitate insurgent movement. Subterranean areas include natural caves,
  basements, manmade underground bunkers, tunnels, holes, and sewer systems. Underground sewers
  and tunnels may also be used in the attack of targets and for egress after an attack. See FM 3-34.170,
  Engineer Reconnaissance, for a discussion of tunnel (and subterranean) detection, reconnaissance,
  maneuver, and destruction.
  Signs of Use of Subterranean Area
  5-124. Certain signs may reveal the often signal or identify that insurgent forces within a certain area are
  using subterranean structures. These indicators include—
  • Movement of insurgents in a specific direction when spotted by aircraft.
  • Sniper fire occurring from areas where there are no obvious avenues of withdrawal.
  • HUMINT reports of subterranean areas
  • Failure of cordons to prevent withdrawal or infiltration of insurgent forces
  • Turned or managed soil far away from places of habitation or daily labor.
  • Operations where insurgents inflict casualties and withdraw without detection or engaging
  COIN forces.
  • The smell of burning wood or food cooking in an uninhabited area.
  • Mounds of dirt, and dirt of different colors, which might indicate digging.
  • Trails to water sources in uninhabited areas that may indicate personnel requiring water.
  Methodical and Coordinated Approach
  5-125. Searching an area where suspected subterranean facilities are located requires a methodical
  and coordinated approach. The size of the surface area and the suspected size of the subterranean
  determine the size and the strength of the unit assigned. The unit is task-organized for subterranean search
  operations, and is divided into five elements: C2, security, search, guard, and reserve. The C2 element
  often remains with the reserve element.
  Narrowing of the Search
  5-126. To detect or locate subterranean, leaders first reduce the geographical area of interest to smaller
  areas of probable locations. Acquiring existing blueprints, maps, imagery, video, aerial photographs,
  and hydrology analysis tools; actively observing for indicators of probable subterranean access locations;
  and questioning the local population as to the existence or specific knowledge of any subterranean.
  Overhead imagery may produce results if the appearance of the surface and vegetation are changed or
  if deductions about substructure can be made from analysis of existing or historical terrain.
  Security
  5-127. Perimeter and flank security is imperative. A slow, methodical search is conducted in the area
  of operations, with each search team systematically searching every square meter. The security element
  moves toward the limits of advance of the search area. Deliberate search techniques emphasize where
  to look for the insurgent locations that provide him with observation, cover, concealment, and an
  egress route.
  Signs of Tunnels
  5-128. Several visual signs help in detecting the actual tunnels. Visual inspections often disclose the
  general area of a tunnel, but not its precise location. The keys to finding a tunnel system are a thorough
  terrain analysis (OAKOC) and an equally thorough physical ground search.