If you watched the Season 7 premiere of Game of Thrones, you know that the Hound saw something ~interesting~ in the fire.
That left a lot of fans confused: If the White Walkers could've just gone around the Wall, why didn't they do it a long time ago?
The answer might lie in the opening credits, of all places. Here's a shot of the Wall from Season 6, Episode 1:
And here's a shot from the Season 7 premiere. Notice anything different?
That's right: The entire sea is FREEZING OVER!
So that means that the huge army of wights really could just — you know — walk around the Wall.