Nantucket Shoals is an area of dangerously shallow water in the Atlantic Ocean that extends from Nantucket Island, Massachusetts, eastward for 23 miles (37 km) and southeastward for 40 miles (64 km); in places water depth can be as shallow as 3 feet (0.91 m). Depth soundings are unpredictable due to the constant change caused by strong currents, which are rotary rather than reversing. The shoals lie just off of a major transatlantic shipping lane. Numerous ships have been wrecked here, most recently and notably the oil tanker Argo Merchant in December 1976. Until 1983, the edge of the shoals was guarded by the Nantucket Lightship.