Adam Soule, the Chief Scientist for Deep Submergence at Woods Hole, says it’s this meticulous attention to detail that’s helped Alvin avoid having even a single serious accident after more than 5,000 dives. “We’re not developing prototypes,” Soule says. “All the technology we develop has to be bulletproof, so there’s a lot of engineering that’s done before anything makes it onto the sub.” Still, there have been some close calls. Only a few years after Alvin was commissioned, a mechanical failure on its carrier ship caused it to fall into the ocean and it began to sink with three crew members inside. The crew narrowly escaped, but it took a year to recover Alvin from the bottom of the ocean.

Alvin has been in service for nearly six decades, but due to regular teardowns and rebuilds, the submarine piloted by Strickrott has little more than a name in common with its progenitor. For the philosophically inclined, Alvin calls to mind the Ship of Theseus, an ancient thought experiment in which the boards of a ship are torn out and replaced one by one until nothing of the original remains. Over the years, Alvin has been upgraded several times so it can carry researchers ever deeper into the ocean, spend more time at depth, and carry more samples plucked from the seabed. But until its most recent remodel, Alvin’s depth rating only gave it access to around two thirds of the seabed. There was a lot more ocean to explore.

Alvin’s current upgrade is the second and final phase of an overhaul that began nearly a decade ago. Funded by a $40 million grant from the National Science Foundation, the first phase laid the foundation for subsequent improvements that would extend the sub’s maximum depth from 4,500 to 6,500 meters, which is deep enough to cover 99 percent of the world’s seabed. By the time that phase was finished in 2013, many of Alvin’s components were already rated to the full 6,500 meter depth, including the sub’s personnel carrier, a cramped titanium alloy sphere. But Alvin has had to wait to venture into those depths until after the final improvements were completed during the second and final phase of the upgrade this year. “Back in 2013, about 70 percent of the sub was replaced,” says Strickrott. “We knew that we were going to operate for a period before we finished the last bits and pieces, which is what we’re doing now.”

Once engineers at Woods Hole have put the finishing touches on Alvin in the spring, it will undergo a rigorous testing process to prepare for its first dive to 6,500 meters. The first tests of the full vehicle will be uncrewed and will demonstrate that Alvin can run its life support systems for 24 hours without creating any harmful gases that would endanger its passengers. Next, a three-person crew will spend 12 hours inside Alvin on the shore to test its life support system again. If everything goes well, the Navy will give the Woods Hole team the go-ahead to begin tests in the water.

Next September, Alvin will be transported by ship to Puerto Rico, where it will begin its first wet tests. Over the course of a week, Alvin and its crew will dive progressively deeper in roughly 500 meter increments. By the end of the week, Alvin will have reached its maximum depth and touched the seafloor in the abyssal trenches off the Puerto Rican coast. If the tests go well, the Navy will officially authorize Alvin for regular crewed expeditions to that depth, and the submarine will spend most of the next five years in the water around the US conducting scientific research until its dragged back to Massachusetts for its regular tuneup.