Ever since the release of Super Mario Maker for the Wii U, dedicated creators have wanted a way to craft their own overworlds to string together levels. The Mario series' overworlds are rarely just ways to package levels—they also include bonus levels and secrets. Super Mario Maker 2 finally got World Maker in an update this week, and though its functionality is somewhat limited, players have found there is a way to hide a level in it.
Thanks to this handy explanation from the r/MarioMaker subreddit, the steps for hiding a level in Super Mario Maker 2's World Maker are pretty easy to follow. The level in question has to be the last one in the world, either a Castle or an Airship (optional in Worlds that aren't the last one in your creation). It then has to be placed on a water, lava, or air tile, which you can swap to any regular path tile to according to the World theme. Unfortunately, World Maker doesn't have a great tutorial, but switching tile types works the same as changing enemies and items in Course Maker.
Follow these steps, and the level should be hidden when you enter that world. A demonstration clip included with the Reddit thread also shows off that there's a neat animation revealing the hidden stage. It's pretty similar to the dramatic reveal of the entrance to the Valley of Bowser in Super Mario World.
The World Maker, though a nice surprise, is still pretty constrained given that each level in it needs to behave like a regular Super Mario Maker 2 creation. You can't carry over power-ups between levels, and Nintendo hasn't added the ability to create hidden exits. Even with branching paths, warp pipes, and this neat option to hide the final level, there's not really a way to make a truly hidden stage or rig up a whole secret World. Super Mario Maker 2 Worlds have to be completed with the final Castle or Airship in each one, so even if you can't see it from the start, you'll need to find it and you'll know it's just stashed away in a seemingly empty tile.
Nintendo has said that this is the "final major update" for Super Mario Maker 2, so it seems unlikely that we'll see any additional features added to the World Maker past this point. Then again, maybe it just means we won't see the addition of more course parts and enemies, leaving other tweaks on the table.
Given that Nintendo satisfied a ton of fan wishes all at once here—adding the Koopalings, Phanto, and a nod to Super Mario Bros. 2—maybe some more World Maker features wouldn't be enough to rank as a "major" update. If this turns out to be Super Mario Maker 2's swan song, so be it, but fans would definitely be left even happier if they had more tools to make their Worlds feel like real-deal Nintendo creations.