(CBS) — Nearly 100 years later, rare video of one of Chicago’s greatest tragedies has been discovered.

On Saturday, July 24, 1915 the passenger ship Eastland rolled over while tied to a dock in the Chicago River, killing at least 844 people on board. The boat was preparing for a Lake Michigan cruise to Michigan City, Indiana for a Western Electric employee picnic.

Jeffery Nichols, a graduate student at the University of Illinois at Chicago, came across the footage while perusing a European website last week. He had been working on his doctoral dissertation.

“It was a ‘Holy cow’ moment. I was shocked. I was surprised,” he tells CBS 2’s Pamela Jones.

The film, from the EYE Film Instituut Nederland, includes footage of rescuers walking on the overturned hull to help survivors and a second clip shows the attempt to remove the ship from the Chicago River.

The Eastland Disaster Historical Society says the film humanizes the tragedy like never before. The organization says it’s the first time footage of the disaster has been found.

“The film footage discovered by Mr. Jeff Nichols is the single most important discovery from research over the past 100 years for those who are interested in the history of the Eastland Disaster,” Ted Wachholz, Executive Director and Chief Historian of EDHS said in a news release. “Over the past 16 years, EDHS has pursued numerous leads, driven hundreds of miles, sent emails, and made phone calls all while responding to reports of possible film footage of the Eastland Disaster. All of these previous leads have ended without any results. Late Friday, however, Mr. Nichols was gracious enough to share his discovery on our Facebook page, which was a very unselfish act. All of us at EDHS and many more who follow us have gotten chills upon viewing this incredible footage, and we know that thousands more will experience the same.”

Surviving passenger Libby Hruby was just 10 years old when the disaster happened. She returned to the Chicago River in 1995 to commemorate the 80th anniversary.

“I remember all the commotion, and the people, the sirens,” she said then.

She died in 2004.

The film clips can be viewed at www.eastlanddisaster.org/news.