Here's just some of the improvements:
1) The sub Alvin stays buoyant with the help of syntactic foam, now rated to go four miles underwater. The foam is composed of billions of glass air bubbles the size of powdered sugar, encased in resin.
2) A life-support system includes a scrubber that removes carbon dioxide from the air and tanks of extra oxygen.
3) An 18 percent larger, seven-foot-diameter personnel sphere holds a pilot and two scientists (one more scientist than any other research sub). To ensure that the sphere can withstand the 10,000 pounds per square inch of pressure at four miles deep, builders modeled the stress it would experience underwater at half a million different locations across its surface.
4) A new hard drive can hold the 1 to 1.5 terabytes of data scientists expect to collect on each dive—an improvement over the VHS tapes and laptops previously used.