These screenshots come from when I was working on the multithreaded lighting system. I could click with the mouse and carve tunnels with giant spheres out of the ground and then WASD around to see lighting and triangulation artifacts. Other keys would create or remove various patterns in the voxel world to more easily test problematic situations.
I had a lot of fun just flying around at this point, carving tunnels like a sandworm through the underworld.