This Achievement is for finding the secret area and Pirate Ship on
Mission 01 Stage 02 : The Bridge.
Play the stage as normal from beginning to end. Once you get to the end , don't collect the exit sign , but make your way all the way back to the beginning of the stage. keep going left until you find a pirate ship behind you. Jump on the ship to get the Achievement and also the hidden item which is an extra mine.