Yeah, I said change, but it's gain. That's what I was thinking, but the words came out differently. From the Wiki page "with 54,200 feet (16,500 m) of accumulated vertical climb".
To understand it fairly, watch the doc. Remember when James Earl Ray escaped from prison in 1977? This is the inspiration for the race's creation. JER made it 8 miles in 55 hours, then he was caught. A local named Gary Cantrell said "hrumph ( I'm paraphrasing ) I could do better than that"...thus the Barkley's was born.
Just know the experts in this field call it the most challenging run in the US. Not the world, there are way tougher ones in Europe.
I did the Presott Half without training and they tout it as one of the toughest. But that was all road and gravel surfaces. My ass was thoroughly cooked when I finished...but I finished running across the line. Too smoked to even sit. Just kept moving....to the bar.
And there were water and aid stations all over.
Mind you, I grew up about 90 minutes from where the Barkley's is. I have hiked/camped all over that stuff. To set out alone without tech...nope. That ain't me.