You'll need 80 Green Stars to unlock this level. You'll have to use Toad to collect one of the items here. You have 100 seconds to complete this level. The level takes place underground. Go back and to the left and ground pound the patch of dirt, where the tiles are broken. This makes the first Green Star appear. Go forward and get the Double Cherry from the item block. Use the two-person platform to cross the gap. Falling into the gap is instant death. On the right side, beat the Goombas and stand on the button with the Toad symbol for the stamp. Grab another Double Cherry and go left. Beat the Fire Piranha, grab another Double Cherry and take the four-person platform to the second Green Star. Drop down to the right for the third Green Star. Go back and right, past the blue lava bubbles to the flagpole.
The stamp is of Small Toad running after a Mushroom.