The Aille river, which runs throughout Co. Mayo, has a particular feature which is very unusual—namely an underground course. The river extends from the Partry mountains and disappears into the Pollatoomary cave in Aille. It resurfaces again in Bellaburke (Michigan State University, 2009). The river drains the Partry mountains and rises in Glenmask, ten miles south of Westport (Coleman, 1950). Lorna Siggins (2008) of the Irish Times writes that, “The river runs underground some 10km southeast of Westport, and reappears three kilometers further on at Bellaburke…​”

The (2009) Michigan State University source recounts that in 1975, potholers (i.e., explorers of caves) (Dictionary.com, 2023) from England descended over 110 feet into the Pollflanagan cave (which is another name that locals had for Pollatoomary cave according to the Mayo County Geological Site Report). The potholers believed that it was possible to descend into the Aille cave, and based on their diving experience they told the locals that the fish below ground had not developed eyes because it was so dark at those depths. It was said that the rise of the river Aille is deeper than any other in Europe, but the potholers were unable to prove their claims because they lacked the proper equipment at the time. Right where the river reemerges, there is a large rock located just before a small bend. The name of that rock is Cloggernayle, and sizeable sea birds called cormorants come to catch their fish there (Michigan State University, 2009).

In 2008, a Polish diver named Artur Kozlowski dived the cave at the Aille river and managed to reach a depth of 103 meters (338 feet) (Michigan State University, 2009) making the Pollatoomary cave the deepest underwater cave in Ireland or Britain (Siggins, 2008). Prior to this, the deepest that any diver had descended into the cave was 33 meters—and that person was Martyn Farr in 1978 (Michigan State University, 2009). Farr believed that he had met the floor of the cave during his dive, but later he was quoted saying that “Obviously it was just a ledge” (Siggins, 2008). Pollatoomary had initially been located by Mr. Farr and it was in fact Mr. Farr who had trained Mr. Kozlowski (Siggins, 2008). Siggins, of the ‘Irish Times’ (2008) quotes Mr. Kozlowski as saying that the dive involved “swimming in cramped confines in poor visibility through the Pollatoomary cavern.” Kozlowski used a “trimix” of gases to stop himself getting the “bends” on what he called his “journey to the centre of the earth” (Siggins 2008). He said that “There is an awful lot of wriggling in tight corners,” and that “It really can only be compared to space exploration, in that we have climbed the world’s highest mountains, explored parts of the ocean, but the subterranean is the last frontier” (Siggins, 2008). Kozlowski went on to say, “Life forms do exist at these depths.” “Imagine when you are swimming along, or negotiating very tight spaces, and suddenly an eel careers into your mask, jaws open and eyes piercing. The eel is harmless, but the effect when you are concentrating is pretty electric” (Siggins, 2008).