Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound. Covering 147 mi (380 km), the Bay forms New England's largest estuary, which functions as an expansive natural harbor, and includes a small archipelago. This bay is mostly within the state of Rhode Island and Providence Plantations, but small parts of it extend into Massachusetts. Of over thirty islands in the Bay, the three largest ones are Aquidneck Island, Conanicut Island, and Prudence Island.