While submarines had great destructive potential in the hands of the military, they also had great potential for peaceful purposes as they allowed humanity to begin to explore the enormous depths of the ocean. Already even before World War II, construction had begun on such vehicles, which were designated DSV (Deep Submergence Vehicle). After the end of the war, construction resumed with several nations including the United States and the former Soviet Union having several by the later half of the 20th century.

Japan also had its own series of DSVs, which it used to explore the depths of the ocean around its four main islands such as the area around the Japan Trench. Among the important vehicles used were the Wadatsumi-No.1 and Wadatsumi-No.2. For a good price, the agency that owned the two DSVs would lend them out for scientific exploration.

The Wadatsumi-No.1 was a DSV that impressive capabilities. It was capable of diving for long periods up to 10,000 meters and for situations in which the submarine's powerful lights could not shine into the dark depths of the ocean, the submarine was capable of firing a torpedo that when detonated would light up the area for a period of time. The submarine had a crew capacity of three people with its driver, an assistant and a passenger.

Sometime in the later half of the 20th century, a series of incidents began to occur around the main islands. After an entire island in the Ogasawara Trench sank beneath the waves in one night, the Japanese Weather Service sent the Wadatsumi-No.1 to the area by a Maritime Patrol Boat to help in the investigation. Piloted by Toshio Onodera and with Professor of Geology Tadokoro as one of the passengers, the Wadatsumi began several journeys down into the ocean floor to figure out the cause of the sudden sinking of the island. After having several narrow escapes while investigating the Ogasawara Trench and then stopping by at the Japan Trench, the submarine and its crew ascended to the surface of the ocean. There, the ship that transported the sub picked it up and all headed back to Tokyo with all of them, particularly Tadokoro thinking very hard about what they had just seen.

Sometime later, Tadokoro requested the use of the Wadatsumi-No.1 again but by this time, it was busy with a project for another company and as a result and with government backing, a French DSV, the Kermadic was used instead. However, when the first of the earthquakes that would do the unthinkable in terms of sinking the entire country of Japan began to occur, then the Defense Agency immediately requisitioned the use of the two Wadatsumi submarines.