The north-east coast of Australia is fronted by the world's longest coral reef system, which stretches in a north/south direction between latitudes 9° and 24° South. Although its outer limit is at distances of between 20 and 120 miles from the coast the enclosed waters are relatively shallow and contain many islands, detached reefs and shoals. The shipping lane between the coast and the Great Barrier Reef is known as the Inner Route. Torres Strait, its northern entrance, although extensive is shallow throughout and encumbered by reefs and islands to such an extent that only one route through it has been lit. The alternative routes are shallow or tortuous.