Mount Vernon is owned and maintained in trust for the people of the United States by the Mount Vernon Ladies' Association, a private, non-profit organization founded in 1853 by Ann Pamela Cunningham. The Association is the oldest national historic preservation organization in the country, and it has always been in the forefront of the restoration field. It is directed by a Board of Regents, comprised solely of women, who represent over 30 states. A Board of Advisors of prominent citizens from across the country meets twice a year to provide additional input on Mount Vernon's governance. The estate, gift shops and dining facilities are operated by about 550 employees and over 400 volunteers. Mount Vernon is the most popular historic estate in America and is open 365 days a year. Mount Vernon does not accept grants from federal, state or local governments, and no tax dollars are expended to support its purposes. Primary sources of income are revenue from the retail and dining facilities, ticket sales, and donations from foundations, corporations, and individuals. The estate, gardens and farm of Mount Vernon totaled some 8,000 acres in the 18th century. Today, roughly 500 acres of this historic estate have been preserved 16 miles south of Washington, D.C., on the banks of the Potomac River. Visitors can see 20 structures and 50 acres of gardens as they existed in 1799. The estate also includes a museum, the tombs of George and Martha Washington, Washington's greenhouse, an outdoor exhibit devoted to American agriculture as practiced by Washington, the nation's most important memorial to the accomplishments of 18th-century slaves, and a collection which features numerous decorative and domestic artifacts. Mount Vernon welcomes an average of 1,000,000 visitors each year.