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Map of the Stellwagen Bank sanctuary

Stellwagen Bank National Marine Sanctuary is an 842-square-mile (638-square-nautical-mile) federally protected marine sanctuary located at the mouth of Massachusetts Bay, between Cape Cod and Cape Ann. It is known as an excellent whale watching site, and is home to many other species of marine life.

The heart of the sanctuary is Stellwagen Bank, an underwater plateau stretching 19 miles north-to-south, and six miles across at its widest, near the southern end. The bank is, on average, 100 to 120 feet (30 to 40 meters) below the surface, while surrounding waters to the west are over 300 feet deep (100 meters) and to the northeast as deep as 600 feet (200 meters).

The steep sides of the plateau cause deep-water currents to rise up when they hit the bank; this upwelling brings with it nutrients and minerals from the bottom, feeding the local ecosystem and attracting larger animals to the area. Whale watchers can frequently see humpback whales, Minke whales and, occasionally, fin whales.

Stellwagen Bank owes much of its existence to the last major ice age. 25,000 years ago, the Laurentide ice sheet advanced over the eastern United States, pushing in front of it large amounts of earth and rocks. The southern margin of the glaciers formed local geographical features including Cape Cod and Stellwagen Bank. Originally, the bank was above water, but gradually subsided over time as the post-glacial rebound subsided.

In the 17th century it was observed that the area made for excellent fishing. Large cod and tuna were frequently caught in the area, and whaling ships caught many whales in the area.

In 1854, the US Navy sent Lieutenant Commander Henry Stellwagen to survey and map the area. It was known that there was a bank in the area, but its extent and shape were not known. Sounding could show ships how close they were to the dangerous waters of Boston Harbor, and so better maps were needed.

Prior to Stellwagen's survey, it was believed there were two small banks in the area: one just to the north of Cape Cod, and one in the middle of the entrance to Massachusetts Bay. Stellwagen showed that they were part of one large bank. As a result, the Navy named the bank after him in 1855.

In 1992, Congress designated the area a National Marine Sanctuary.

• Stellwagen Bank National Marine Sanctuary homepage