While the mounds had been studied by some travelers, professional excavation under the evolving techniques of archeology did not begin until the 1930s, under the administration of President Franklin D. Roosevelt during the Great Depression. The Works Progress Administration (WPA) sponsored large-scale archaeological digs at the site between 1933 and 1942. Workers excavated portions of eight mounds, finding an array of significant archeological artifacts that revealed a wide trading network and complex, sophisticated culture. On June 14, 1934, the park was authorized by Congress as a national monument and formally established on December 23, 1936 under the National Park Service.