While exploring Teyvat in Genshin Impact, you might find yourself stumbling across an abyssal domain that is seemingly inaccessible. When you first discover Taishan Mansion, it is completely submerged underwater. To unlock it, you'll need to do some puzzle-solving, which the video above can walk you through. Or, if you'd prefer, read on below about how to unlock Taishan Mansion.
When you find Taishan Mansion, just off to the right of the entrance is a stone tablet which gives you your first hint of how to unlock it. After reading the description, Paimon suggests that there might be something that will let you drain water out of the lake the mansion is submerged in.
Almost directly across the lake from the mansion (to the north east), there's a gang of Hilichurls and an Abyssal Mage that Paimon points out to you as suspicious. Fight and defeat them--plus two more waves of enemies (Slimes followed by more Abyssal Mages)--to unlock an obelisk overlooking the lake.
Activating the obelisk drains the lake slightly, revealing another obelisk in the center of the lake. To activate this one, you need to break rocks surrounding the lake and guide the geo spirits that follow you from them to the stone lamps. After filling all the lamps with spirits, the second obelisk unlocks and you can drain the lake further.
And that's all there is to it! Once the lake is fully drained, Taishan Mansion and a Precious Chest will be right there for the taking. Also be sure to grab the Geolocus handing out at the bottom of the lake too.
As for the mansion itself, don't worry if you then can't open it. Taishan Mansion only unlocks after hitting Adventure Rank 27. You'll want to run this domain regularly once its available though, as it contains valuable Character Talent level up materials that rotate daily.