CUDAHY, Wis.  The deep is legendary for inky darkness. William Beebe, the first person to eye the abyss, called it perpetual night.

The darkness is matched by the intense pressure. Four miles down, it amounts to nearly five tons per square inch. That is too much even for Alvin, the most famous of the world’s tiny submersibles, which can take a pilot and two scientists down to a maximum depth of 2.8 miles.

But a new submersible is being built here, and even the process of construction seems a rebuke to the darkness. The work lighted up a cavernous factory with fireworks on a recent visit. Hot reds and oranges burst into showers of spark and flame as blistering metal began to yield to the demands of the submersible’s design.

“Amazing,” Tom Furman, a senior engineer at Ladish Forging, said after a big press bore down on an 11-foot disk of hot metal, making the delicate manipulation look as easy as rearranging a gargantuan pat of butter.