Maro Reef (Hawaiian: Nalukākala - "surf that arrives in combers") is a largely submerged coral atoll located in the Northwestern Hawaiian Islands. It was discovered in 1820 by Captain Joseph Allen of the ship Maro, after whose ship the reef was named. With a total area of 747 square miles (1,935 km²), it is the largest coral reef in the Northwestern Hawaiian Islands. It contains 37 species of stony coral. Unlike most atolls, the coral extends out from the center like spokes on a wheel. Located about 850 miles (740 nmi; 1,370 km) northwest of Honolulu, Hawaii, Maro Reef contains about 1 acre (4,000 m²) of dry land which itself can be submerged depending on the tides.