The World Surf League's annual Tow Surfing Challenge took place earlier this month with the world's top big-wave surfers both male and female taking on waves 40 to 50 feet high. Here's an example (taken at another time of year) of what these intrepid surfers come to tackle in the town of Nazaré, about an hour and 20 minutes north of Lisbon .

(credit: Luis Ascensio)