Ulysses Moore is a series of adventure books written by the Italian author Pierdomenico Baccalario.

[1] The plot of the series centers on the fictional village of Kilmore Cove and its Doors of Time.

Once opened, it leads to a distant time and space specific to that door and it can only be reopened when all of the travelers who passed through it return.

However, four keys are needed to open that one, making it the most difficult door to access.

This particular door leads to a cave where there is a lake and a boat called "Metis" used to take the Travelers to their destinations.