Dark inside hole or cave ... how to?

I would like to achieve effect like it's on this image. You can see that "dark" or "black" is nicely transitioning into the hole.

How would one achieve this effect?

 

You could use a texture.

 

Baked lighting could also solve this

 

Concerning the yellow and black spiral hole thing, I think that is a seperate 3d object from the enviroment. Because I see dark, pixelated, razor square teeth artifacts a bit under it.

If so:

It's a possibility, that the dark shadow leading down to the dark hole, and getting a slight bit darker, as it gets closer to the hole, could be a shadow painted onto the yellow and black spiral hole texture, using the colour black, with a slight transparent gradient added to it. You can do that in photoshop, or Gimp.

Or, it's possible, that they made the yellow and black swirly spiral texture. And then made another seperate texture, to create the blended gradient shadowy part. And then, in the layers section of photoshop, they put the gradient shadow texture onto the top layer, above the yellow and black spiral texture, and then added a filter effect, to the gradient shadow texture, and then flattened or combined both textures together, creating what you see in the pic above.

What I explained above might sound complicated, but it is much easier than it sounds.