Ireland is situate in the Atlantic Ocean and separate from Great Britain by the Irish Sea. Half the size of Arkansas, it occupies the entire island except for the six counties that make up Northern Ireland. Ireland resembles a basin—a central plain rimmed with mountains, except in the Dublin region. The mountains are low, with the highest peak, Carrantuohill in County Kerry. The principal river is the Shannon, which begins in the north-central area, flows south and southwest and empties into the Atlantic.