The Northern Mariana Islands, together with Guam to the south, compose the Mariana Islands. The southern islands are limestone with level terraces and fringing coral reefs;the northern islands are volcanic, with active volcanoes on Anatahan,Pagan and Agrihan. The volcano on Agrihan has the highest elevation in the islands at 3,166 feet (965 m). About one-fifth of the land is arable, another tenth is pasture. The primary natural resource is fish, some of which are endangered species, which leads to conflict. Also, development has created landfills which have contaminated groundwater on Saipan, which might contribute to disease. Anatahan Volcano is a small volcanic island 80 miles (130 km) north of Saipan and is about 6 miles (10 km) long and 2 miles (3 km) wide. Anatahan began erupting suddenly from its east crater on May 10, 2003, at about 6 p.m. (0800 UTC). Since then it has continued to alternate between eruptive and calm periods. On April 6, 2005, approximately 50,000 cubic meters (1,800,000 cu ft) of ash and rock were ejected, causing a large, black cloud to drift south over Saipan and Tinian.