Lough Neagh (, ) is a freshwater lake in Northern Ireland. With an area of 392 square kilometres (151 square miles), it is the largest lake in the British Isles and ranks among the forty largest lakes of Europe. Located twenty miles (30 km) to the west of Belfast, it is approximately twenty miles (30 km) long and nine miles (15 km) wide with an area of 392 square kilometres (151 square miles). It is very shallow around the margins and the average depth in the main body of the lake is about 9 m (30 ft); although at its deepest the lough is about 25 metres (80 ft) deep.
Of the 4550 km² catchment area, around 9% lies in the Republic of Ireland and 91% in Northern Ireland; altogether 43% of the land area of Northern Ireland is drained into the lough, which itself flows out northwards to the sea via the River Bann. As one of its sources is the Upper Bann, the Lough can itself be considered a part of the Bann.
Five of the six counties of Northern Ireland have shores on the Lough: Antrim, Armagh, Down, Londonderry and Tyrone. Towns and villages near the Lough include Antrim, Crumlin, Randalstown, Toomebridge, Ballyronan, Ballinderry, Moortown, Ardboe, Maghery, Lurgan and Magherafelt.
Traditional working boats on Lough Neagh include wide-beamed 16–21 ft clinker-built, sprit-rigged working boats and smaller flat-bottomed "cots" and "flats". Barges, here called "lighters", were used up to the 1940s to transport coal over the lough and adjacent canals. Up to the 17th century, log boats (coití) were the main means of transport, some of which are as old as 6,400 years. Few traditional boats are left now, but a community-based group on the southern shore of the lough is rebuilding a series of working boats.
In the 19th century, three canals were constructed, making use of the lough to link various ports and cities: the Lagan Navigation provided a link from the city of Belfast, the Newry Canal linked to the port of Newry, and the Ulster Canal led to the Lough Erne navigations, providing a navigable inland route via the River Shannon to Limerick, Dublin and Waterford. The Lower Bann was also navigable to Coleraine and the Antrim coast, and the short Coalisland Canal provided a route for coal transportation. Of these waterways, only the Lower Bann remains open today, however a restoration plan for the Ulster Canal is currently in progress.
Lough Neagh Rescue provides a Search & Rescue service twenty-four hours a day. It is a voluntary service with funding being provided by the District Councils bordering the Lough, its members are highly trained and are a declared facility for the Marine Coastguard Agency who co-ordinate rescues on Lough Neagh Maritime and Coastguard Agency.
Lough Neagh was widely assumed to be owned by the state, but in 2005 it publicly emerged that it is the ancestral property of the Earl of Shaftesbury. This may have serious implications for planned changes to state-run domestic water services in Northern Ireland, as the lough supplies 40% of the region's drinking water and is also used as a sewage outfall (in a system only permissible through British Crown immunity).