Plain cover franked with 1938 4d light blue & red orange GVI issue (SG 158) tied by fine oval SEA FLOOR BAHAMAS cancel dated May 6 1940 with 'POSTED IN THE WILLIAMSON PHOTOSPHERE AT BOTTOM OF SEA, NEAR NASSAU IN TH BAHAMAS' cachet & 'Bahamas - Williamson Undersea Expedition' marking both on front. Addressed to USA.