Pisces V

Pisces V is a deep-submergence vehicle (DSV) of the Pisces class built in 1973.

Pisces V is a type of manned submersible ocean exploration device, powered by battery, and capable of operating to depths of 2,000 metres (6,600 ft), a depth that is optimum for use in the sea waters around the Hawaiian Islands. It is used by scientists to explore the deep sea around the underwater banks in the main Hawaiian Islands, as well as the underwater features and seamounts in the Northwestern Hawaiian Islands, specifically around Loihi Seamount.

In 1973, Pisces V took part in the rescue of Roger Mallinson and Roger Chapman, who were trapped on the seabed in Pisces V's sister submersible Pisces III. In August 2002, Pisces V and her sister Pisces IV discovered a World War II Japanese midget submarine outside of Pearl Harbor which had been sunk by the destroyer USS Ward in the first American shots fired in World War II. In 2011, marine scientists from HURL celebrated the 1,000th dive of Pisces V.

The advantage of having two is that it allows preparation for an emergency. While one of the submersibles is conducting its dive, the other remains at readiness should there be an emergency, needing to be boarded on ship and hurried to the site of the problem. Such an emergency could include the submersible becoming tangled in fishing nets or entrapped in rocks or debris on the ocean floor. In such cases, the second heads to the rescue. There are also research experiments where it is advantageous to use the two vessels together.

...
Wikipedia