American photographer John Ernest Williamson (1881-1966), a pioneer in undersea photography, designed a chamber called the Williamson Photosphere, from which he could take pictures of undersea creatures. Between 1939 and 1941, with the cooperation of the Bahamaian postal authorities, Williamson operated an undersea post office in his photosphere, canceling covers with a "Sea Floor, Bahamas" oval date stamp. These covers were considered novelties by collectors of the period.
Many of the covers included a 1938 4p stamp with a design featuring tropical fish in a coral landscape, based on a photograph by Williamson.
Today there is a considerable interest in the Sea Floor covers. They sell for prices ranging from $30 to as much as $200.