There is usually water in the bottom of the deeper craters. When a series of craters is formed, running into one another, you get a very uneven floor that appears in lengthwise section thus:--
The dotted line is the ground level: the uneven line is the course that would be taken by a man walking along the bottom of the chain of craters, and keeping in the centre. Actually, of course, (on patrol) one would not keep in the centre where the crater contained water, but would skirt the water by going to one side of it. The “bridges” are important, as they are naturally the easiest way across the craters; a bombing patrol, for instance, could crawl over a bridge, without having to go right down to the bottom level, and (which is more important) will not have a steep climb up over very soft and spongy soil. These bridges are the “lips” of the larger craters where they join the smaller; looking at a crater-chain _in plan_ X is a “bridge,” whereas Y and Z are “lips” rising above ground level.
This crater-chain being understood, the system of sentries is easily grasped. Originally, before mining commenced, our front line ran (roughly) from left to right along Rue Albert up 80 A Street and along to the top of 76 Street in a straight line. Then began the great game of mining under the enemy parapet and blowing him up; and its corollary countermining, or blowing up the enemy’s mine galleries before he reached your parapet. Such is the game as played underground by the tunnelling companies, R.E. To the infantry belongs the work (if not blown up) of consolidating the crater, whether made by your or an enemy mine, that is to say, of seizing your side of the crater and guarding it by bombing-posts in such a way as to prevent the enemy from doing anything except hold his side of the crater.
For instance, take a single crater, caused by us blowing up the German gallery before it reaches our parapet. If we do nothing, the enemy digs a trench into the crater at A, and can get into the crater any time he likes and bomb our front line, and return to his trench unseen. This, of course, never happens, as we dig a sap into the crater from our side, and the result is stale-mate; each side can see into the crater, so neither can go into it.