Last week, Walter, Peter, and Olivia went to the school where Walter did experiments on Olivia as a child. It was locked up and the combination to the door was 5 - 20 - 10. Walter made a point of saying that he can't remember the significance of why they always used the same combination. You can see the quick scene below:
At first, I thought it could be coordinates, but then I remembered it's the year 2010 and the idea of May 2010 or 5/20/10 being an important date popped into my head. Perhaps, it's the date that the two worlds are set to collide. DUM-Dum-dum!! One question though, if Walter used the other world to get his son back after he died in our world, what did William Bell want it for?