Super Mario World/Valley of Bowser 2

This level is far by the hardest Underground, it has the Secret to Valley Fortress which leads to the Back Door to Bowser's Castle....On the 3rd Screen of the level, there is a spot you have to run before the Yellow Ground squishes you, when your at the end and the ground has came up, jump on it and go left for a LONG or short walk till you see a Key, thats your time to fave the Hardest Fortress!