World 6-1 (Super Mario Bros. 2)
From the Super Mario Wiki
This level starts like any level from World 2 does: as the desert is explored, numerous enemies and obstacles must be beaten, such as quicksand and swarms of Cobrats and Pokeys. Then a fort is entered, where 21 jars are seen. Only one of them has the key that is necessary to open the door showing Green Birdo's whereabouts. Another jar leads to a 1-Up Mushroom.