We don't actually know the name of the underground city with the coffin and the pool, but calling it Old Amsterdam seems like a safe bet. It was first discovered by clipping out of bounds, so at first it was thought to be an unfinished part of the game, but we've since found a normal way to get down there, one intended by the devs, so this is a very intentional easter egg that people are meant to find. For the community to figure out what's going on, though, we need to know how to get down there.



Start in the Underground Bazaar. It's unlocked by talking to the Oldheads.
When you first enter, you'll see some spots to wallride to take you onto some suspended rails. That's where you want to start.



Once you're on the big, circular rail, there will be another wallride that leads to a pipe with a rail on top. That's the next step.



From here, you can just barely make out the vent by the big JD ad. That's our goal, and we need to get there by riding these rails to the end and then getting up on the barely-visible ledges along the walls.



Once you're up on the ledges, you'll still need to jump around these wide columns to make it onto the next ledge over until you're up against the JD ad we saw before. Once you've done this enough times, a simple double jump will be plenty to make it across these gaps, but a double jump with a boost jump after is the safest way to make sure you can clear the columns. If you brush up against the wall, it'll kill your momentum and send you down like a rock, so do your best to make sure you're getting plenty of air so you don't have to start the line over.



Once you're next to the ad, you'll be next to the vent. If you used a boost jump to get onto the last ledge, that will even kick the vent down, showing you you can do inside. Slide through to reach the secret, underground city, and then explore on your own.