The largest digital environment in "The Expendables" was the Devetashka cave (Located near Lovetch, Bulgaria), which had to be digitally recreated because of the environmental concerns of filming explosions in the real cave.
Using over 5,000 photographs taken at different times of day, the cave was accurately illuminated and rendered down to crevices and moss. The cave's interior containing a concealed nuclear vault was subjected to several changes up to two months before the effects' delivery date based on editorial input, including modifications to its scale.
The railcar used within the cave was also digitally built alongside a practical version, but the digital versions versatility saw it largely replace its practical counterpart. The scene featuring the Expendables and prisoners trapped in the cave were shot entirely on Chroma key with the cave background inserted later.
