Whatever it is, I'm doubting that "swim 55 miles" would appear on your list.
Article link here.
A Seattle area man, Tyler Patterson, in celebration of his 40th birthday, decided to swim the 55 mile perimeter of Lake Washington. He managed to do, relying on short stops every three hours for a brief break to eat, drink, or to get a quick rub-down. Only once in 37 hours did he pause for more than a couple minutes (he stopped for almost an hour after swimming essentially nonstop for 27 hours). Amazing. In doing so, raised money for the Experimental Educational Unit, a school at the University of Washington for young children with disabilities.
What I find most remarkable about this is that he started training for this last October with swims of just 1,000 meters in length. To put this in perspective, when I was training for triathlons, the majority of my swim workouts were about 3,000 meters in length -- and that was for swims of only 1.2 and 2.4 miles in length. To go from workouts of just 1 kilometer in length to swimming nonstop for 55 miles less than a year later... like I said, amazing!
Equally impressive is that, including the rests, he averaged close to 1.5 miles per hour throughout the swim. If you were to talk to experienced triathletes or swimmers--whether recreational or competitive, they will tell you that swimming a mile in under 30 minutes is an accomplishment. It's something that you don't just go out and do unless you've been swimming competitively for a while. Kind of like running a 4-hour marathon -- nothing to scream and shout about, but a milestone the "middle of the packers" shoot for nonetheless. To think that someone managed to maintain this pace for a day-and-a-half simply leaves me speechless.