Taishan Fu in Genshin Impact is a closed domain in the realm of Serenitea, which you can unlock by solving a simple puzzle.

Once you reach the required Adventure Rank, which is at least 26, you will see it marked on your map and available to visit!

How to solve the Taishan Fu domain puzzle

The gates are underwater, so to enter, you need to find a way to lower the water level.

How to lower the water level:

First, ascend to the main pedestal marked on the map below. Here, you will find a locked altar and several enemies. Keep fighting until all enemies are defeated. When the last one is defeated, you can interact with the altar (a glowing switch) to slightly lower the water level.

After that, the central platform will open up, revealing several stone lanterns and another locked altar (switch).

Now, look around for glowing Geocrystals. Break one of them with your sword, and you will notice a small speck of light floating around you. You need to bring these specks to the lanterns on the central platform. You can find the crystals near the water at the base of the cliffs surrounding the lake.

Once you light up all three lanterns, you can interact with the central switch, further lowering the water level. After that, a special chest will appear near the entrance to Taishan Fu, and you can finally unlock the entrance to the domain!