Looking at my D/T grid, I notice that I've never found any cache with T4.5 or T5. I searched for those caches, and found that: T5 means kayaking, and T4.5 means tree climbing.
Climbing a tree is definitely out of my comfort zone, but Geocaching is all about exceeding the comfort zone.
But before attempting a very high climb, I gotta practice. Seeing this cache is a T3, I choose this one as my first tree climb.
Georick402 helped me identify which tree it is, then I started the climb. As long as I keep three of my four limbs stable on big branches, tree climbing isn't too scary. Moving from branch to branch, I spotted the cache within 3 meters up. The experience ended sooner than I expected.
I'm told that Georick's AIR mail is "ten times higher". I guess it's still out of my comfort zone until I can get more practice.