On 30 July, the USS Indianapolis, the flagship of the Fifth Fleet, was steaming unescorted from Guam to Leyte.  She had completed a high speed unescorted run across the Pacific four days earlier, delivering some was material to Tinian and now was set to rejoin the fleet.

Sinking

At 00:15 30 July she was hit by two Type 95 torpedoes from the I-58, causing massive damage. The bow was blown off by the first torpedo and the second struck near the powder magazine, with the subsequent explosion splitting the ship to the keel.  The Indianapolis, top heavy due to added armaments, settled by the bow and rolled over 12 minutes later, and killing over 300 of her 1196 man crew. The rest went into the water.

While the US Navy plotted the intended course, when a ship's arrival time came it was assumed a large vessel such as the Indianapolis would make it safely to port. Thus, she was removed from the plot and despite the watch officer knowing she had not arrived, no report was made up the chain of command. Much later, declassified reports showed at least three stations received a distress signal but they were ignored, as were decoded messages from the I-58 reporting the sinking of a large vessel along the Indianapolis' course.

Rescue

As a result, the sailors were left to fend for themselves for 3 and a half days until a PBY discovered the survivors in the water and dropped rafts. LCDR M Doyle, the plane's commander, landed and rescued 56 survivors. He and his crew lashed men on the wing, making the plane unflyable.  Seven ships proceeded to the scene and picked up the remaining survivors. 

Of the 800 or so that went into the water, 316 survived. The rest succumbed to exposure, lack of food and water, and shark attacks. Some men formed large groups to help fend off shark attacks. According to survivors, others swam off into the distance, never to be seen again. 

Aftermath

The Indianapolis’s commander, Captain Charles McVay, was the only officer ever to be court-martialed for the loss of a ship during wartime in U.S. Navy history. He was found guilty of "hazarding his ship by failing to zigzag" and  not guilty of having failed to promptly issue orders to abandon the ship after the torpedo attack. The court-martial was controversial, and the court recommended clemency. 

McVay had not been warned of Japanese submarines in the area and ordered to "zigzag at his discretion, weather permitting." Commodore James Carter, commander of Pacific Fleet’s advance headquarters, had told him before leaving Pearl, "Things are very quiet." The Japanese “are on their last legs, and there’s nothing to worry about.” His request for a destroyer escort had been denied. Survivors contended that poor visibility made zigzagging inadvisable.  In addition, the CO of 1-58 testified that zigzagging would not have prevented the sinking. Admiral Nimitz remitted McVay's sentence and he eventually retired as a Rear-Admiral, a Tombstone Promotion. His feelings of guilt eventually resulted in his suicide in 1968.

Later analysis showed that even in McVay had zig-zagged at least one torpedo would have hit the Indianapolis. Hearings held by the Senate Armed Services Committee resulted in Congress voting in 2000 to exonerate RADM McVay for the loss of the Indianapolis. 

An exploration led by the late billionaire Paul Allen discovered location of the lost cruiser in 2017. In 2018, the crew of the USS Indianapolis was awarded a Congressional Gold Medal.

The cargo it delivered to Tinian? About half of the world's supply of enriched uranium.