Snorri relates in the Prose Edda that Skjöldr ruled the country that we today call Denmark. Skjöldr had a son named Friðleifr who succeeded him on the throne. Friðleifr had a son who was named Fróði who became king after Friðleifr, and this was at the time when Caesar Augustus proclaimed peace on earth and the Christian figure Jesus was supposedly born. The same peace ruled in Scandinavia, but there it was called Fróði's peace. The North was so peaceful that no man hurt another, even if he met his father's or his brother's killer, free or tied. No man was a robber and a golden ring could rest on the moor of Jelling for a long time.
King Fróði visited Sweden and its king Fjölnir, and from Fjölnir he bought two female slave giantesses named Fenja and Menja who were big and strong. In Denmark, there were two big mill stones which were so big that no man was strong enough to use them. However, the man who ground them could ask them to produce anything he wished. This mill was called "Grótti" and it had been given to Fróði by Hengikjopt.
Fróði had Fenja and Menja tied to the mill and asked them to grind gold, peace and happiness for himself. Then he gave them neither rest nor sleep longer than the time of a song or the silence of the cuckoo. In revenge Fenja and Menja started to sing a song named the "song of Grótti" (the poem itself) and before they ended it, they had produced a host led by a sea-king named Mysing. Mysing attacked Fróði during the night and killed him, and left with rich booty. This was the end of the Fróði peace.
Mysing brought Grótti as well as Fenja and Menja and asked them to grind salt. At midnight, they asked Mysing if he did not have salt enough, but he asked them to grind more. They only ground for a short while before the ships sank. A giant whirlpool (maelstrom from Mal -mill and ström -stream) was formed as the sea started rushing through the centre of the mill stone. Then the sea begun turning salt.
Read more about this topic: Grotte