Some of the big adventures that make the list include mountain biking Porter Trail in Utah, a route that is well known for being very challenging and technical, and climbing Denali, in Alaska. While that mountain isn't nearly as tall as giants found in the Himalaya, it's fickle weather, massive prominence, and high latitude do indeed make it a significant undertaking. Hikers will also find the famous Snowman Trek in Bhutan has made the list, as has running Inga Rapids in the Congo, which has only been accomplished once or twice ever to my knowledge.
While I won't give away the rest of the list, I will say there are options for extreme skiing, scuba diving, spelunking, and much more. In fact, Nat Geo has covered just about every outdoor sport in this run-down, including surfing and highlining too.
The title for this article is a bit sensationalists. Don't get me wrong, each of the entires onto the list is indeed an epic adventure with some big challenges to overcome. But to call these options the "most extreme" on the planet is a bit misleading. For instance, there are more challenging mountain biking routes than the Porter Trail and while Denali is an impressive undertaking, there are certainly mountains more difficult, dangerous, and demanding. Still, it is an interesting list to read for anyone looking for ideas on their next major outdoor undertaking.
Check out everything that Nat Geo recommends here.