Japanese seemed to develop Ponape, Caroline Islands into a submarine baseaccording to the U.S. Joint Intelligence Committee. Daily summary dated 3 April 1942

An item reported that the Japanese seemed to develop Ponape, Caroline Islands into a submarine base. The J.I.C. commented that Jaluit was the major submarine base and since the US Navy attacked the Marshall Islands the submarine activities decreased and that the Ponape are was considered as to be more secure.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 114 dated 3 April 1942.