Go to small building with 2 windows by the broken wall.

Go outside to the left between the lumber pile and the stairs, then run back and forth from the stairs to the wall, all the way each time.

If you do it right you should get in the wall, once you are in turn around and don't move, just aim.

Go slowly forwards and to the side and once you feel yourself start to fall in, go the way you're falling and explore.