BTS 1: an extruded cave
I started with a plane. Then I extruded one of the sides, quite randomly. And I started repeating. Extrude, extrude, extrude. Fill some. Extrude some more.. and this was the shape I ended up with. Here you can already see some rocks and a plane for the water I added later.
BTS 2: add moss.
And then for the fun bit! I added moss. In this case random bits of greeny stuff near the waterline and wherever I wanted something special using Blender's Scatter tool.
BTS 3: lots of moss!
Using particle systems I added more moss! Various types and sizes, scaled randomly and with shaders adjusted to all kinds of blueish colors to fit the scene. For this I used the Scatter plugin found on Blendermarket.
Version 1: first render!
Didn't like it. Too bright.
Version 2: it's dark now.
Turned off the sun. Added some moonlight and stars. This is better! I added a reference yellow light to indicate where I might want to build a fireplace. I later abandoned that plan, but it gave me some context and size to work with.
Version 2: added god rays
I wanted some cool atmospheric effects with sunbeams (or moonbeams), and I felt lazy, so I added a sunlight, a cube with volumetric lighting and a ehm.. cheese grater? A thing with random holes in it, to substitute for a cloud deck as I didn't want to find or create a perfect cloud texture, and this would do just fine.
Version 3: added a model
I originally planned to keep this file for a while until I found a shoot that would suit it, but while discussing the earlier versions with Julia, I thought it would actually be nice to re-use a photo from one of our earlier shoots in this picture. So I did. I recolored it to match the scene somewhat, made a cutout and added it to the scene. This is starting to work!
Version 4: add fireflies!
... ehm, no. That didn't work.
Final render BTS
Here you see the various bits that made up this render; the cave with model, the various fog layers. The 'cheese grater' on the left is actually something to block some of the sunlight, so I had more interesting light lines to play with. In the background I used a texture of stars (provided by Alessandro Viaro on Unsplash)
I added some more atmospherics, and some extra dust in photoshop, and done!