This is one of the images from my diploma project "Gameworlds", which deals with the translocation of real people into fictional worlds. The idea was inspired by computer games, thus the project title.
The project consists of five compositing's, each at 11811 x 8350px, so the pictures could be printed in 100x70 cm at 300 ppi.
I dressed and photographed the actors and composed them with 3D elements (in this case the entire submarine room and the tentacles), multiple other photos, and a bit of digital painting in Photoshop, which is my main application.
In this picture I used Cinema 4D to make the submarine, ZBrush for the tentacles, Realflow for the water from the ceiling, and the rest is Photoshop.
After making a few sketches of my idea, I started building the submarine control room in Cinema 4D, using pre-made 3D people just to mark the positions of the actors and the devices, and also to determine the scale of everything else, because "man is the measure of all things" :)
From here on it was basically all about making everything look more complex and cool, adding detail to everything, optimizing the way the objects look, creating better textures, and so on. This is the part that took the most time. Here is a rendering from somewhere in-between and final rendering: