This is in the south of Benthic Kelp Beds in Mount Maelstrom. It's a hole in the seabed, covered with kelp. When you're just below the kelp there's an interact prompt. If you click it there's no name for the interactive object and the description which pops up just says the kelp is amazingly lush.
Does anyone know what it's for or why it's there? I'm guessing it's part of a legendary collection, but I'm not sure.
(I did try searching the Wiki, but there's nothing on the Benthic Kelp Beds page and without a name I don't know what to search for to find the interact point itself.)
"You can run like a river, Till you end up in the sea,
And you run till night is black, And keep on going in your dreams,
And you know all the long while, It's the journey that you seek,
It's the miles of moving forward, With the wind beneath your wings."