Looks like this article or section is a stub and missing info. You can help out by expanding it.
The Underground is a shop in Palia accessible at night from 6:00 PM to 3:00 AM. It's run by the villager Zeki, and features a static register shop, a rotating selection of furniture and decor not craftable in each set in Furniture Making, access to The Tower, and Zeki's Wondrous Machine.
Location
This shop has three entrances: one is in the basement of Zeki's General Store hidden behind a bookshelf, and the other is along Kilima's southern shore beneath the Remembrance Garden, disguised as an entrance to the sewers. A third entrance can be found in Reth's Storeroom located in the basement in Ormuu's Horn Inn.
Once you complete the associated Wares in Low Places quest, Zeki will give you a key which unlocks the entrance from his store.
Each night in Palia there is a small rotating stock of furniture and decor available for purchase with Gold in Zeki's Grimalkin Underground. These include furniture infused with Flow like plumbing and electronics, rugs, and paintings.
Here is a list of all possible items with their Underground prices: