A large rift appears at Faron Wetlands, so you go southeast to investigate.
How to Reach Faron Wetlands
To reach Faron Wetlands, go southeast from Hyrule Castle Town, along the dirt road. When the path splits, go east, then when it splits again, go south to enter Lake Hylia.
Things to Do in Lake Hylia
While in Lake Hylia, if you haven't already, you can visit the Great Fairy Fountain in the cave in the center of the lake to meet the Great Fairy. She can increase the number of accessories you can wear, for a fee. The first increase costs 100 rupees, and the next is 300 rupees, then 500 rupees, and the final upgrade is 1000 rupees.
West of the Great Fairy Fountain, there is a rift. You can check on the glowing spot in the rift to have Tri create an entrance, then check on the entrance to go into Stilled Lake Hylia.
Continue on to Faron Wetlands
Swim to the east side of Lake Hylia, where there is a waterfall. Use echoes, such as water blocks, to reach the top of the waterfall. From there, go east on dry land, and climb down the rock wall. Check on the waypoint to add it to your map.
Now that you're here, you can try to learn more about the Rift in the Faron Wetlands.