mer de glace
/ˌmɛː də ˈɡlas/noun
A glacier or frozen sea; specifically (in form Mer de Glace) the great glacier on the north-west slope of Mont Blanc, near Chamonix, in the French Alps.
Origin
Early 19th century; earliest use found in Maria Edgeworth (1768–1849), novelist and educationist. From French mer de glace, lit. ‘sea of ice’ from mer sea + de + glace ice.