National Park Mystery Spot 3 is the summit of Mount McKinley, the crowning peak of the Alaska Range and the landform centerpiece of Alaska’s Denali National Park and Preserve. This enormous mountain, which is more massive than Mount Everest (and rises more steeply), is known to locals and climbers around the world as Denali, an Athabascan Indian name that means “High One” or "Great One."

And a very high and great one it is! Denali's South Summit, the highest of its two significant summits, tops out at 20,320 feet, (nearly 6,194 meters). That makes Denali the tallest mountain in North America.

These three clues were provided:

About 6,774 double-time military paces
25th in the series
One might stay there forever

The first two clues relate strictly to the mountain’s name and elevation. Mount McKinley is named for Ohioan William McKinley, the 25th President of the United States (and thus, “25th in the series”). A double-time military pace is 36 inches (three feet), and 6,774 of those is 20,322 feet. Even if you took the regular 30-inch military pace -- or any kind of pace, for that matter -- you would have gotten a large enough number to be on the track of the answer.

The third clue relates to the many climbers who have died on Mount McKinley and whose remains are still on the mountain. There are dozens of them buried in the snow, lost in crevasses, or lodged in inaccessible places. There is even a body that was left on the South Summit with no immediate plans for retrieval. Yes indeed; if one visits Mount McKinley and runs out of luck, even if one has made it all the way to the top, “one might stay there forever.”

Incidentally..... in the pre-Google age I would have included the clue "Leon Czolgosz" just for grinnies.