The Melanesian Border Plateau covers an area 1000 by 200 miles along the north-eastern edge of Melanesia, facing the Central Pacific Basin, with an average depth of 1500 fm (2700 m). It trends E-W, but is broken up into a series of narrow ridges and troughs en échelon trending NW-SE, each approximately 250 miles long and 100 miles from ridge crest to crest. The troughs rarely exceed 2200 fm (4000 m) in depth. Some are closed basins and others open out in a funnel shape, sloping gradually down into the Central Pacific Basin (2700 fm or 5000 m). Although the Border Plateau is bounded by the 'Andesite Line' there is an anomalous absence of any belt of deep trenches on the Basin margin. The échelon ridges of the Plateau are mostly less than 1000 fm (1800 m) deep. They are capped by a few small volcanic islands and a large number of slightly submerged (10-15 fm or 18-27 m) atolls of dead, 'drowned' corals. Alexa Bank is a characteristic example. The cause of coral death is a mystery; vulcanism and foul upwelling are possible explanations.