Tasha Wilson was walking her dog at Portelet when she noticed something surfacing.
She later posted the footage on Facebook in the hope that Islanders could solve the mystery.
Almost 100 people have responded to the post, some saying it is a whale, while others have suggested it could be a basking shark.
Some have said it could be a plank of wood or a detached shellfish storage box.
Do you know what it could be?
