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PBS News Hour / YouTube

The world's largest instrument exists deep inside the Luray Caverns in the Appalachian Mountains, and it's 400 million years in the making, Scientific American reports. It's called the Great Stalacpipe Organ, but it's unlike a typical organ, which forces air through pipes to create music — this instrument rhythmically strikes the cave's stalactites to create beautiful sounds.

How it works: Large, rubber mallets sit next to the 37 stalactites. When the organist strikes a key on the instrument, the corresponding mallet strikes the stalactites, all coordinated via a hidden mechanical device that receives electrical signals from the organ. The organ's inventor sanded down 35 of the 37 stalactites to perfect their tone (two didn't need it) and the 3.5-acre cave lends natural acoustics for the songs.

Because the acoustics are not uniform throughout the cavern, it can be difficult to play by ear. So an automated system — using a plastic sheet with holes in it that rotates around a metal drum — plays songs similar to a player piano. When the metal meets the drum, corresponding stalactites sound off.

Go deeper: A combination of Mother Nature's work, time and a skilled mathematician helped create the Great Stalacpipe Organ. The large, echoey chambers of Luray Caverns have formed naturally over the past 400 millions years. Calcium-rich water droplets eventually formed the large stalactites that hang from the cave's ceiling and now act as an integral part of this instrument. Back in 1954, mathematician and electronics engineer Leland Sprinkle visited the caverns. As was customary during these tours, the guide would strike the stalactites to show visitors how each one gave off a unique sound. That moment sparked an idea in Sprinkle's head — create the Great Stalacpipe Organ, which he did in three years.

Go deeper

Exclusive: Houston mayor to lead Black mayors group

Houston Mayor Sylvester Turner speaks during a private funeral for George Floyd. Photo: Godofredo A. Vásquez/Pool/Getty Images

The mayor of the city where George Floyd was raised is taking over a group that represents 500 Black mayors in the U.S. amid national pressure to revamp police departments.

Why it matters: Houston Mayor Sylvester Turner will become the new president of the African American Mayors Association as municipalities across the country examine police reforms and deal with the economic fallout from the pandemic.

Delivery industry sees biggest monthly job losses in more than 20 years

Data: U.S. Bureau of Labor Statistics; Chart: Axios Visuals

The pandemic's biggest job winner is losing steam.

Driving the news: People who deliver packages to businesses and homes — classified as "couriers and messengers" by the Labor Department — saw the industry's biggest monthly job losses in more than 20 years in April.

FDA authorizes Pfizer COVID vaccine for 12- to 15-year-olds

Photo: Gabby Jones/Bloomberg via Getty Images

The Food and Drug Administration authorized the emergency use of the Pfizer-BioNTech coronavirus vaccine for 12- to 15-year-old adolescents, the agency announced on Monday.

Why it matters: The emergency authorization marks a critical milestone in the push to get more Americans vaccinated and fully reopen schools for in-person learning this fall.