Lough Mask (Irish: Loch Measca) is a limestone lough (lake) of 20,500 acres (83 km²) in County Mayo, Ireland, north of Lough Corrib. Lough Mask is the middle of the three lakes, which empty into the Corrib River, through Galway, into Galway Bay. Lough Carra flows into Lough Mask, which feeds into Lough Corrib through an underground stream which becomes the River Cong. Lough Mask is the sixth largest lake, by area, in Ireland. The eastern half of Lough Mask is shallow and contains many islands. The other half (Upper Lough Mask) is much deeper, sinking to a long trench with depths in excess of 50 metres. Lough Mask has a mean depth of 15m, and a maximum depth of 58m. Its water volume of 1.3 km³ is only surpassed by Lough Neagh's 3.5 km³ in Ireland and it is the largest lake, by water volume, in the Republic of Ireland.