Once you're done, head back out and on to the cemetery where you will once again go underwater. Follow these directions to get to where you have to be: Take 5 steps forward, one to the right, another one forward, move to the Left, take 2 steps Forward, one to the Right, another step Forward. Now turn left , two more steps Forward, one to the Right, another Forward then to the Left, 4 more steps Forward, one last move to the Left, 2 steps Forward, one final move to the Right and finally one more step onward!
You'll get to this room with another Noisette puzzle. Take note of the words, Europe, Pont Neuf, Blanche and Chateau Rouge. Now take note of the metro stops. Europe is 3, Pont Neuf - 7, Blanche -2, and Chateau Rouge 4. Enter the numbers 3724 to get another key.