That tree, which you can see from Firelink is indeed Great Hollow.
You can go to an edge of Firelink and look down. You can see Blighttown. To be exact, you can only see the stench from there, as it clouds your view. When you descend Blighttown itself, you can always look above and see sky. It's just that terrible stench obscuring the light, which makes you think it's inside a cave. Also the entrances make it look like so, but it is indeed on the outside.
I've always thought that the archtrees, with the Ash Lake itself, are below the ground and that the whole world actually sits on their branches, with the Ash Lake being the lowest part of the world.
You can go to an edge of Firelink and look down. You can see Blighttown. To be exact, you can only see the stench from there, as it clouds your view. When you descend Blighttown itself, you can always look above and see sky. It's just that terrible stench obscuring the light, which makes you think it's inside a cave. Also the entrances make it look like so, but it is indeed on the outside.
I've always thought that the archtrees, with the Ash Lake itself, are below the ground and that the whole world actually sits on their branches, with the Ash Lake being the lowest part of the world.