The Shinkai 6500 (しんかい) is a manned research submersible

Submersible

A submersible is a small vehicle designed to operate underwater. The term submersible is often used to differentiate from other underwater vehicles known as submarines, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is...

 that can dive up to a depth of 6,500 m. It was completed in 1990 and has the greatest depth range of any manned research vehicle in the world. The only manned expedition to have gone deeper was the dive of the Trieste bathyscaphe in 1960. However, the vessel could not navigate along the bottom of the sea bed. The Shinkai 6500 is owned and run by JAMSTEC (Japanese Agency for Marine Earth Science and Technology) and it is launched from the Yokosuka.

Two pilots and one researcher operate within a 73.5 mm thick titanium pressure hull with an internal diameter of 2.0 m. Buoyancy is provided by syntactic foam

Syntactic foam

Syntactic foams are composite materials synthesized by filling a metal, polymer or ceramic matrix with hollow particles called microballoons, "syntactic" meaning "put together"...

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Three 14 cm methacrylate resin view ports are arranged at the front and on each side of the vehicle.