Downtown (which can also be called the Underground City or the Underwater City depending on the tide) is a secret area on the game Super Mario 64 and Super Mario 64 DS and the level in the game, Wet-Dry World. To get there, the player has to launch Mario into a very tall fence at maximum high tide (it is inaccessable at any other tide). Once within the cage, you must swim through a tunnel with partially closed gates as fast as you can before Mario loses all of his air reserve (his health). The city has the 8 Red Coins hidden and the Power Star required to get all six stars for that level (7 in the DS version). To get the star, you have to get the Vanish Cap (or Power Flower for Luigi in the DS remake) and become invisible, then run through the fence. To retrieve all of the red coins you have to look around by jumping on top of the buildings, balancing on thin walls, and breaking Bricks, all while strategically utilizing the Crystal Tap and the ! Switch (there are times when you have to use the Metal Cap or the power flower for Wario to get to and activate these important items). There are some enemies here as well, but they are fairly easy to avoid. Other Items include green and blue ! Blocks. The only help you can get besides from an item is from a lone Bob-omb Buddy that gives you advice (in the DS version, he will tell you where the 8 red coins are).
The city is barren and somewhat small. It is unknown what species of creatures inhabited this place, though many are quite sure they were Mermushrooms. The latter is not confirmed but the city is quite modernized, so it could be a small pun the Lost city of Atlantis. This place is currently being patrolled by a small fleet of Bowser's army. These special operation members include only a few Skeeters and a couple (broken) Fire Spitters.
Downtown is, as latter told, a modernized, yet small town having only 5 inaccessable buildings, plant life, and even a central park with thin stone walls and a statue of a pyramid. Another attraction is the shrine without an entrance that stands in front of the park. It has an opening where you can collect the star after you've gathered all 8 red coins. The closed area with the other power star was possibly something under construction, which could further induce the fact that there was life in the city.