Ro-100 was not one of the big Japanese fleet submarines designed to cross oceans and attack American battle lines. She was a coastal type, sixty meters long, with a crew of fifty, derived from the earlier Kaichu class. On the surface she could manage about 14 knots; submerged, 8. Her four bow torpedo tubes carried a total of eight torpedoes, and she carried light anti-aircraft mounts on deck. Her diving limit was 75 meters. Her job was to patrol the waters around Japan's forward bases - Rabaul, the Solomons, New Guinea - and to intercept Allied shipping that got too close. It was dangerous work with almost no upside. The patrol areas had been picked over by her sister boats, the Allies had begun deploying better sonar, and the losses were mounting fast.

Her first real test came on 14 February 1943, off Port Moresby. Ro-100 sighted an Allied convoy and began a submerged approach. Her commanding officer, concentrating on the target, neglected the standard periscope sweep and did not see the Allied destroyer bearing down on him. The escort dropped more than a dozen depth charges. Leaks opened in the main engine room and conning tower. Both periscopes failed. The boat tilted up at a steep angle, in danger of losing control and plunging past her collapse depth. In desperation, her captain ordered every available crewman forward to the bow torpedo room - fifty men's worth of human ballast, trying to weigh her nose down so she could level out. It worked. She limped back to Rabaul for repairs, and when she got there her officer corps was quietly reshuffled.

Through the summer of 1943, Ro-100 was assigned patrol area after patrol area southeast of Guadalcanal, north of New Guinea, south of Rendova. Most of these patrols were uneventful. The Allies' convoys had learned to stay elsewhere; the submarines that did sight them were often the ones that did not come home. In early July, during her fourth patrol, she ran onto a reef in the Blanche Channel near New Georgia while conducting a night reconnaissance. The collision tore open a fuel tank and damaged two of her bow torpedo tubes. She was diverted to pick up stranded Japanese naval pilots from the 201st and 204th Naval Air Groups at Simbo and delivered them to Buin on Bougainville - a taxi service for airmen whose planes had been shot out from under them. In August an unidentified submarine - probably American - fired four torpedoes at her. All of them missed. That night an Allied patrol plane caught her on the surface off Cape St. George, dropped flares, missed with two bombs, and strafed her as she crash-dived.

On 1 November 1943, U.S. Marines landed at Cape Torokina and the Bougainville campaign began. Japanese forces on the island were cut off from easy resupply by sea. The garrisons at Buin and elsewhere needed food, and submarines like Ro-100 were among the few assets that could still get it to them. On 23 November she loaded rubber cargo containers packed with rations at Rabaul and set out on what would be her seventh and final war patrol. Two days later, just before 19:10 local time on 25 November 1943, she was on the surface in the Bougainville Strait, making for Buin via the northern channel, two nautical miles west of Oema Island, when she struck a mine. The explosion blew her commanding officer and the lookouts on the conning tower clean off the boat and into the water. She began to sink fast. Her engineering officer - the ranking survivor aboard - ordered everyone still below to abandon ship.

Fifty men had sailed on Ro-100. Twelve came home. Survivors in the water began swimming toward the lights of Buin, a few miles to the north. Sharks found them first. The accounts are stark: men who had survived the first explosion, who had gotten out of a sinking submarine in the dark, were attacked as they swam for a shore they could see. Of the crew of fifty, thirty-eight died - most of them not in the initial blast but in the swim that followed. Ro-100 was struck from the Japanese Navy list on 5 February 1944. She rests somewhere on the bottom of the Bougainville Strait, near Oema Island, where the water is warm and the currents carry over reefs that are still being charted. Her rubber cargo containers, the food she was carrying to men who needed it, went down with her.