Sarah Carlson gave the students fair warning about what to expect when they reached the caverns, 80 feet below ground.

“It’ll be pretty chilly down there,” said Carlson, who works in special collections at the University of Minnesota library. “Oh, yeah, you’re not going to have any signal,” she added, as a teenager fiddled with a cellphone. “Sorry about that, guys.”

The students, from Gordon Parks High School in St. Paul, were on a pilgrimage to the underground vaults where the U houses some of its most prized possessions: in this case, the Givens Collection of African-American Literature. But even as they crammed, single-file, into a climate-controlled chamber, the students knew there was an easier way to see what was in some of those boxes.

If only they could get a signal.

For the first time, the U is starting to make collections like the Givens accessible online. It has launched an ambitious project, called Umbra Search, to make it easy to search not only its own collection, but hundreds of African-American archives across the country. All at once.

The year-old website, UmbraSearch.org, “is an amazing resource,” said Catherine Squires, a U communications professor, who has been testing it all semester with classes at Gordon Parks.

For students, she said, Umbra is “a one-stop shop” to explore historical images and voices of the past, from slavery to the Harlem Renaissance to the Civil Rights movement. “It’s a great tool to increase people’s awareness of just how rich that history is,” she said.

At this point, only a small fraction of library collections is online. But it’s only going to grow as libraries find the time, and money, to digitize more material, said Cecily Marcus, the U librarian who directs the Umbra Project.

So far, Umbra has made a dramatic, if spotty, start, with links to more than 400,000 pieces of history — photos, videos, letters and manuscripts — from 500 libraries and museums, including the Smithsonian Institution.

Among the collections are handwritten letters by an 18th-century slave and poet, Phillis Wheatley; 1940s FBI surveillance reports on the activist and scholar W.E.B. Du Bois; and even curiosities like the 1960s Black Panthers Coloring Book (a government hoax, it turns out, to try to discredit the group).

At Gordon Parks High School, students have been exploring the website as part of a class about the school’s namesake, a legendary African-American photographer. “They know that Gordon Parks is some sort of famous guy, but they really don’t know why,” said Squires. Their assignment: to see what they can learn using the archives in Umbra.

At first, many students wonder why they even need it, said Jamie Tomlin, who teaches the class. They often ask: “Why can’t we just use Google images?”

The answer is a lesson in the difference between historical documents and Wikipedia.

What they begin to discover, said Tomlin, are pieces of history “that you can find nowhere else,” or that might be lost in a Google avalanche.

Search Umbra for Gordon Parks, for example, and you find an oral history he recorded in 1964; some of his most famous portraits (Stokely Carmichael, Malcolm X); and links to other African-American photographers who left their mark on the 20th century.

For students who have barely heard of any of them, “it opens up a new world,” said Squires. “All of a sudden, they’re seeing all kinds of things they never would have imagined.”

The U started the project, with about $750,000 in grants, to shine a light on materials that many people don’t even know exist, according to Marcus. “African-American history holds a central place in American history, but it’s very much in the shadows,” she said. That’s why they named the project Umbra, the Latin word for shadow.

Maryama Dahir, who graduated from the U in May, said she discovered the wonders of Umbra while working as a student curator on the project this spring. She spent hours on the site, she said, leafing through the letters of Zora Neale Hurston, Langston Hughes and other pillars of the Harlem Renaissance. “I would lose track of time,” Dahir admits. Not that it was a waste; she turned her research into her senior thesis.

“Everything that I found on Umbra was a surprise,” she said.

At this point, the website is still a work in progress, Marcus admits. It’s officially in “beta test” mode, and the searches can be unpredictable. There’s also a question of where the future funding will come from, she said, since the U wants to keep the service free.

But eventually, Marcus hopes it will become a national resource. “This is a commitment that the University of Minnesota has made,” she said. “It’s not going to go away.”

To Squires, Umbra is one more way to preserve a precious legacy. “If we don’t keep it safe, we can’t hand it down to the next generation.”