So Thor decapitates the finest bull in Hymirs posession and uses the head as a bait.
They row far out and Thor is not impressed about the two whales, Hymir pulls up.
Further out, Thor pulls up the Midgardserpent itself, a giant snake, that surrounds the world.
He wants to kill it with his hammer, but Hymir is so scared, that he cuts the line and the serpent sinks back.
So in the weaving of the hammer you can see the Midgardserpent swallowing the bait.
That saga was depicted very often by the norsemen on runestones, etc.
The heads on the necklace are replicas from Kungsgården, Gamla Uppsala (SE) from around 1000- 1100.
Finest quality work, availiable in 925 Sterling silver and bronze.