Allied POW camps and ship-transports were sometimes accidental targets of Allied attacks. The number of deaths which occurred when Japanese "hell ships"—unmarked transport ships in which POWs were transported in harsh conditions—were attacked by U.S. Navy submarines was particularly high. Gavan Daws has calculated that "of all POWs who died in the Pacific War, one in three was killed on the water by friendly fire". Daves states that 10,800 of the 50,000 POWs shipped by the Japanese were killed at sea while Donald L. Miller states that "approximately 21,000 Allied POWs died at sea, about 19,000 of them killed by friendly fire."