You need 80 gems to unlock this level. I have 117.
Captain Toad finds himself on a small field with a small body of water. There's a plant moving around. Pick it to get a pickax. Use it to break the brick wall in the rocks, then drop into the water. At the bottom are some bricks. Break them to get the first gem. Pull the handle below to get back up. Pick the plant near where you started for another pickax. Go through the bricks behind you. You'll find more pickaxes and some Piranha Sprouts. Walk around the corner twice and find an area where you can drop down. With the pickax, drop down to the right for the second gem. Drop down further and you'll find some Mud Troopers. Head to the left. There appear to be some bricks blocking your path, but you can go behind them. Go up the stairs, then down and you'll find Green Toad. There are two plants in front of him: one has a Mushroom and the other has a turnip. A Mud Trooper will appear on the stairs, so defeat him with the turnip. Go up the stairs and you'll find more turnips and Mud Troopers. Go around the corner and you'll find a lone Piranha Sprout. One of the nearby plants has a pickax. With it, drop down on the side with the Piranha Sprout two floors down and you'll find the wall bricked up. Use the pickax to break it to get the third gem. Drop down to the Star, which is guarded by Mud Troopers. Behind the brick wall on the right are coins and a Gold Mushroom.
Special Challenge: Don't break any bricks.
You can't use the pickax to break any bricks. There are parts between the bricks where you can drop down.