It is situated on a 600-acre peninsula between Castle Hume Lough and Lower Lough Erne. Other large lakes include Lough Erne. They reached upper Lough Erne in ancient times. A second extensive lake system is centred on Lower and Upper Lough Erne in Fermanagh. The next largest is Boa Island, within Lower Lough Erne. The 4th Battalion also carried out waterway patrols on upper and lower Lough Erne. The other UK population in Lough Erne has severely declined. The north-eastern part of the county, however, drains towards the River Erne and Lough Gowna. Columbanus left home to study under Sinell, Abbot of Cleenish in Lough Erne. To the north of Lough Erne, the oldest rocks in the county are found. His mother, Lady Mabel Crichton, was the daughter of John Crichton, 4th Earl Erne. The village came under the control of the Earls of Erne in 1821. He was known to his family and friends as Harry Erne. Originally the term "erne" or "alerion" in early heraldry referred to a regular eagle. Canoeing is also a popular recreational sport on the Erne.