For bats that are year-round residents of temperate areas, underground openings such as caves and abandoned mines are critical resources in winter as hibernacula, and in autumn for swarming behaviours (mating, socialization, etc.). Like many parts of the world, Nova Scotia, Canada, has many underground openings that have not been surveyed for bats. The feature(s) that affect the suitability of these sites for hibernation or swarming is not known. As such, it is not possible to predict which ones are used by local bats (Myotis lucifugus, Myotis septentrionalis and Perimyotis subflavus). Because it was not safe to enter the sites to conduct hibernation counts, our goal was to relate bat activity at the entrance of underground openings during swarming to quantifiable external metrics and to pre-existing data on internal site characteristics. Specifically, our objectives were to 1) identify abandoned mines and caves that are used by bats for swarming and 2) quantitatively characterize factors which best differentiate between underground openings that are used for swarming, and those that are not. We assumed that sites used by bats for swarming were likely also used for hibernation. Acoustic and/or trapping surveys were conducted at 17 abandoned mines and eight caves in Nova Scotia, five of which were previously known to be hibernacula. Results suggest that at least 12 of the 25 sites were swarming sites (including seven newly identified sites). Logistic regression analysis of nine a priori selected models indicated that internal chamber length was the best predictor of swarming activity. Two external variables, degree of shelter at entrance and total length of rivers in landscape, were also important predictors. These variables have the potential to be used as indicators to identify swarming sites at other sites in eastern North America. The swarming sites identified in Nova Scotia should be targeted for monitoring in light of the devastating impacts that white-nose syndrome is having on North American bats.