First, go to forge then choose sandbox. Then, you go to the middle of sandbox.

You should see that one part of the floor is different than the other.

Next, you press up aim at the piece of floor that is different than the other.

Aim at it should be green your aimer. Last,you need to press B and it should disappear.

Remember, that if you go in the hole it will look like if you're gonna die, but you won't, instead you will land safely to the ground and see a huge room with laser (or whatever you want to call them).