As they skate or snack in Bryant Park, visitors might dismiss the stately New York Public Library next door as a dog-eared relic in an age of digital information.

But unbeknown to most of them, 17 feet below ground, in a concrete bunker worthy of the White House, the library is expanding and updating one of the most sophisticated book storage systems in the world.

Since March, after abandoning a much-criticized plan to move the bulk of its research collection to New Jersey, the library has been working instead to create a high-tech space underground for the 2.5 million research works long held in its original stacks.

The books will begin arriving in April, and by the end of spring library officials expect to be using a new retrieval system to ferry the volumes and other materials from their 84 miles of subterranean shelving, loaded into little motorized carts — a bit like miniaturized minecars carrying nuggets of research gold.