Many locations in Hogwarts Legacy are hidden away, and the Underground Harbour is one of them. It's very easy to find but extremely easy to miss. This location is essential in the Hogwarts Legacy story as you need to find something hidden there, so here is a guide on getting to the Underground Harbour in Hogwarts Legacy.
How to find the Underground Harbour in Hogwarts Legacy
The Underground Harbour in Hogwarts Legacy is a small secret area below the castle that contains two chests, an eye chest, and a Field Guide Page. There are two ways you can access the Underground Habour. Oddly enough, I found the Harbour the swimming way, yet there's a way to save yourself from getting completely soaked.
How to access the Underground Harbour from the Boathouse in Hogwarts Legacy
To get to the Underground Harbour by swimming, go to the Boathouse Foo Flame. Start swimming West towards the castle. You will eventually be able to see a waterfall as you approach the castle. Directly to the left of the waterfall is a cave opening with vines hanging over the entrance. Swim through, and you will be in the Underground Harbour.
How to access the Underground Harbour from the Viaduct Courtyard in Hogwarts Legacy
Finding the elevator down to the Underground Harbour is a little faster than swimming. First, travel to the Viaduct Courtyard Foo Flame. As you spawn in, walk directly forward, then keep right. Continue to follow the wall on your right, and you will approach a door in the wall. This is the elevator that will take you down to the Underground Harbour.
The Underground Harbour is a small area, but if you want to find all Field Pages at Hogwarts, you will need to visit it. The chests there will also give you some gold and a couple of items.
For more on Hogwarts Legacy, check out How to enter the Treasure Vault under the Innkeeper's Shed in Hogwarts Legacy here on Pro Game Guides.