Actually, your question refers to Super Mario World. To get to the red switch palace, you must pass Vanilla Dome 2 (the red circle in the water). This is found in the cave area. There are 2 ways to pass this level. You must swim through the water the same way you usually would and then you will come to a spot where you can jump out of the water onto a question mark box. There, you would normally go right. Instead go left and grab the "P switch" and carry it through the wall of coins. Then place it on the ground and jump on it. Run left. The brown blocks will change to coins and you will be able to run through them. You must jump over the first hole and then down the second one. To your left there will ba a key. Pick it up and keep going to your left. You will fall back into the water. Swim to your right and you will see a keyhole. Put the key into the keyhole and then a new path will be opened to the red switch palace.