With the DRC gone, the explorer base dried up. There was no reason to visit the Cavern anymore. For all intends and purposes, it was dead.
But the Guilds, they had their solutions, they Wrote a new Age...we'll call it D'nay for now. But how to get to D'nay?
A small desert town. Las Belleza. A few miles from the site of D'ni. The Guild purchased an abandoned church there, and placed the Book to D'nay, allowing all explorers to know that they were in the right place upon seeing the Church's iconic structure.
Once one arrives at the church (and finds the way in

And that's my idea for how we actually get there. It's plausible, IRL.
Note: The concept of Las Belleza is an old one that I worked on with a friend back at GoAC, as a real-world Link to the Cavern (a town where explorers could buy supplies, find a safe home near the cavern, etc.). The church was partially built and so was a gas station and a few other things for the town. In my mind, there would be two versions of it--the "entry" one (like the Cleft) that I described here, empty of people, and one much more like the City--one set at night. This would be, ICly, how explorers get back the surface--and could also serve for some storyline events, seeing as it's public.

-Kato