Donut Secret 1 is the first underwater stage in Super Mario World. It can be accessed using the secret exit in Donut Plains 1.
It is full of all the three kinds of fishes: Blurp, Rip Van Fish and Cheep Cheep. It's highly helpful to enter this level with Fire Mario. If not, a Fire Flower can be found not far from the start. The first purple pipe leads to a bonus level, which can be navigated with P-Balloons. After taking the first balloon head left, a second balloon is inside a ? block. Then head for the middle and a yellow block will have a 1-up mushroom. Then to the right collect a Dragon Coin and fall to collect another Fire Flower. To take the secret exit grab the P-Switch and swim with it until you see the key hole. Press the switch and inside the ? block is the key.