Upon reaching level 8, you'll unlock access to a challenge related to the Awakening of Mjolnir, Thor's legendary weapon. All you have to do here is pick it up in the crater where he washed up, not far from Salty Springs, while wearing the Thor skin.
The hammer only recognizes its owner. Picking it up means that you're worthy enough to use it. Mjolnir is located right in the center of the crater, visible from the flight over the area.
All you have to do is interact with the item to complete the challenge, which will reward you with a hammer that can be used as an harvesting tool!
Original content by Romain "Zorg" Becquelin.