The Middelzee (Dutch for "middle sea"; West Frisian: Middelsee), also called Bordine, was the estuary mouth of the River Boorne (West Frisian: Boarn) now in the Dutch province of Friesland. It ran from as far south as Sneek northward to the Wadden Sea and marked the border between main Frisian regions of Westergoa (Westergo) and Eastergoa (Oostergo). Other historical names for the Middelzee include Bordaa, Borndiep, Boerdiep, and Bordena.[1] The names like Bordine, mean "border".[2]