Hey!

Thought it would be good to make a guide about the clip down to the mine.

When you're getting out of House of Portals, you'll have to stand where there is marked.

When you're standing there, you want to go close to the fence behind you. Now hold A + S down, and click CTRL. (This will get you out of bounds)

When you're getting behind the fence, you'll have to walk down to the grass and walk forward a bit.

Now look for this:

You'll have to stand in the middle where there is marked. When you're standing in the middle, then crouch, hold S down until you're falling down. When you're starting to fall, quickly hold "W" down.

Then you'll get down to the mine. Remember to walk forward, when you're getting down there!

Also here is a clip, if this didn't help you.