Mount Everest

noun2

1. •
   A mountain in the Himalayas, on the border of Solukhumbu district, Koshi, Nepal and Tingri County, Shigatse, Tibet Autonomous Region, China; the world's highest mountain above sea level.

   • The mountains of Asia are the loftiest in the world. Mount Everest, one of the Himalayas, is more than five and one-half miles high,—a mile higher than Mount Tupungato! — 1863
2. •
   (figurative) An endeavor that is very demanding yet rewarding; also, something which is the highest achievement, challenge, etc.; the epitome, the pinnacle, the ultimate.

   • After this rather tricky problem let's climb the Mount Everest of all writing problems. I mean, of course, the world-famous, forbidding peak of U.S. income-tax prose. — 1958