At a. Uttrick peered through his binoculars from the deck of the minesweeper U. By the pale light of a waning moon, the American sailor spied something unusual piercing the glassy skin of the Pacific Ocean less than two miles south of the entrance to Pearl Harbor. Hours before swarms of enemy aircraft descended out of the blue in a sneak attack upon the United States naval base on Oahu, five Japanese midget submarines were already lurking beneath the ocean surface to join in the assault on Pearl Harbor.
X-class submarine - Wikipedia
A midget submarine also called a mini submarine is any submarine under tons,  typically operated by a crew of one or two but sometimes up to 6 or 9, with little or no on-board living accommodation. They normally work with mother ships, from which they are launched and recovered and which provide living accommodation for the crew and support staff. Both military and civilian midget submarines have been built. Military types work with surface ships and other submarines as mother ships. Civilian and non-combatant military types are generally called submersibles and normally work with surface ships. Midget submarines are best known for harbor penetration, although only two World War II boats, the British X-craft and the unsuccessful Welman submarine , were specifically designed with this in mind.
On 29 May five large Japanese I Class submarines rendezvoused some 35 nautical miles northeast of the entrance to Sydney Harbour. Before daylight the next morning an E14Y Glen float plane launched from one of the submarines, I , and crewed by Warrant Flying officer Susumo Ito and Ordinary Seaman Iwasaki, flew a daring reconnaissance mission over the harbour, twice circling the cruiser USS Chicago before flying off to the east. The aerial intrusion was observed and reported but it did not initiate any special harbour defensive measures being implemented. Many mistakenly believed it an American floatplane conducting a routine training flight.
Transported on board large I type submarines, the midgets were launched near the entrance to Pearl Harbor the night before the attack was to begin. One, spotted trying to enter the harbor before dawn, was attacked and sunk by USS Ward DD in the first combat action of the as yet unopened Pacific War. Another, the Ha , unsuccessful in its attempts to penetrate Pearl Harbor, drifted around to the east coast of Oahu and was captured there the day after the attack. Four of the five Pearl Harbor midget submarines have been found. That sunk by Ward was located by the Hawai'i Undersea Research Laboratory in late August in deep water, some five miles off the entrance to Pearl Harbor.