The entire area is riddled with underground river channels and caves carved through the karst limestone, so the River Cong, which connects the two great loughs, performs a peculiar trick: it disappears almost entirely underground for much of its course, surfacing briefly, vanishing again, and reappearing at the edge of the village before emptying into Corrib. In dry periods, the riverbed can be completely exposed while the water continues its journey out of sight below.

Cong has accumulated a considerable weight of history on this already unusual geography. An Augustinian abbey was founded here in the twelfth century, and its ruins remain largely intact at the centre of the village, including a roofless but well-preserved church and some finely carved Romanesque doorways. The site had earlier ecclesiastical associations going back to Saint Feichin in the seventh century. Cong is also the location of the discovery of the Cross of Cong, an early twelfth-century processional cross of oak covered in gilded bronze and decorated with filigree and enamel work, which is now held at the National Museum of Ireland in Dublin. The cross was made to enshrine a fragment believed to be from the True Cross, and it ranks among the finest surviving examples of Irish Romanesque metalwork. The area around the abbey also features a series of fishing lodges and covered channels built in the nineteenth century, an elaborate attempt to channel the underground waters into usable fishing streams, though the project largely failed because the water simply drained away through the porous rock before the fish could be tempted in.