This is an abandoned limestone mine with about a mile of passages. In just the last few years we have been trapping large horizontal openings like this by placing traps perpendicular to the gates. In this way we take advantage of the swarming behavior and often the traps are more effective.Some plastic sheeting constricts the entrance and encourages the swarming behavior. This technique would not work for a spring emergence trapping effort when bats do not swarm. In this case the traps would be placed parallel with the dripline.