If you fill all terrain with a solid rock then during gameplay after a while it just disappears, leaving only bunker modules. But if you add some background, then everything seems ok. Scene size is just 2.5 Mpix. Another mystery?

Try making the first pixel (top-left) air. For some reason the game removes (parts of the) terrain if the top-left pixel is anything but air.

Oh thanks. I can sacrifice just a single pixel ))