I-52, code-named Momi ( Japanese for "evergreen" or "fir tree") was a Type C-3 cargo submarine of the Imperial Japanese Navy used during World War II for a secret mission to Lorient, France, then occupied by Germany, during which she was sunk.
She is also known as Japan's "Golden Submarine", because she was carrying a cargo of gold to Germany as payment for matériel and technology. There has been speculation that a peace proposal to the Allies was contained onboard the I-52 as well, but this is highly unlikely on two counts: there is no evidence of the Japanese government being interested in peace proposals or negotiated settlements at this early stage in the war, prior to the summer of 1945, and the Japanese kept an open dialogue with their diplomatic attachés via radio and diplomatic voucher through Russia, and had no need for long and uncertain transfer via submarine.
Also interesting is that 800 kg of uranium oxide awaited I-52 for her return voyage at Lorient according to Ultra decrypts. It has been speculated that this was for the Japanese to develop a radiological weapon (a so-called "dirty bomb") for use against the United States (the amount of unenriched uranium oxide would not have been enough to create an atomic bomb, though if used in a nuclear reactor it could have created poisonous fission products).