On the fascinating site Letters of Note:
At 11:00 a.m. on August 9th, 1945, just a minute before the second atomic bomb in the space of three days was dropped on Japan, a B-29 bomber named The Great Artiste quietly dropped three canisters from the sky. Inside each of the canisters, alongside a shockwave gauge designed by American physicist Luis Alvarez, was an unsigned copy of the following letter.
The letter, written by Alvarez and two fellow scientists, was addressed to Japanese nuclear physicist Ryokichi Sagane —a man with whom Alvarez had previously worked at Berkeley — and pleaded with him to inform his ‘leaders’ of the impending ‘total annihilation’ of their cities.
The letter reached Sagane a month later after being found 50km from the centre of devastation: Nagasaki.
Alvarez and Sagane met again 4 years later, at which point the letter was finally signed.
Read the letter and transcript on Letters of Note