This Achievement is for finding the secret area on
Mission 03 Stage 01 : The Cave.
Play the stage as normal from beginning until you get to the 2nd Checkpoint. You have to make sure you are in Ninja Form so you can double jump. At the ledge of the 2nd checkpoint jump to the left and do a double jump to land on a hidden platform. Now keep going left until you see a hole in the floor. Drop down in this hole to be transported down to the hidden area. The hidden area contains 2 extra lives (1Up x 2)
Friendly tip : If you wait at the bottom for just under a minute a Kenpo fighter will spawn in. Kill him and when you go back up there will be 2 more extra items (1 x 1Up and 1 x Ki)