The purpose of air locking is to recover portions of the sealed fire area or the entire area in a manner that will prevent increasing the oxygen content in unexplored areas to avoid an explosive atmosphere or rekindling or intensifying the fire when a suitable air lock has been erected, a proper organization and proper and adequate equipment and material provided and all other necessary arrangements are completed, an oxygen breathing apparatus crew, fully equipped for the work at hand and supported by a fully equipped reserve crew should enter the air lock through a man door and remove the seal. After the seal has been removed, an oxygen breathing apparatus crew, with another crew in reserve, should advance and explore to the point where the next air lock is to be erected. General conditions should be observed by the exploring crew, temperature readings taken, an air sample collected to check previous analysis, measurements made for material required to construct the inby seals of the next air lock, then return to the fresh-air base. An apparatus crew or crews, with a reserve crew at the fresh-air base, should then construct a temporary seal with a door in it at the place previously selected for the next air lock, erect necessary temporary seals in crosscuts or other openings on the intake side and on the parallel entry or entries on the return side opposite the point selected for the air lock to insure resealing of the inby area, and examine any unexplored parts of the isolated area for possible fires. All crews should then be withdrawn from inside the air locks. Next, a seal on the return side should be opened by an apparatus crew: the air-lock doors on the intake side should be opened and air admitted to reventilate the area inside the air locks. Stoppings should be erected in open crosscuts on the return side to advance fresh air to the last crosscut which should be left open to provide a return. The quantity of air should be so regulated that the return will be kept below the lowest explosive limit. After the newly explored area has been reventilated, fresh-air men should erect a tightly constructed stopping with a door in it a suitable distance outby the one previously built to form the next air lock. Advances as described above should be made by successive blocks until the entire area is recovered As the work progresses, frequent analyses should be made to determine the composition o] the atmosphere within the sealed area The oxygen should be kept under control and within safe limits at all times. It is imperative that the oxygen be kept as low as possible at all times by limiting the infiltration of air to the sealed area as the work progresses.