That's Austrian adventurer Michael "Mich" Kemeter slacklining across the gorgeous Green Lake in Austria.
It took Kemeter 35 attempts in 20 days back in September 2013 to achieve the feat - and once he did, he set a new world record (beating his old one) of slacklining a distance of 250 meters.
You can see Kemeter's attempts as well as beautiful shots of Lake Green in the video clip above and read all about it over at Redbull.