We had a similar giant hole in Mladost 1 for more than 3 years. Most of the time there was no visible work activity whatsoever, no change at all month after month. I have no idea why was that necessary, but eventually they finished the construction.

http://bg.wikipedia.org/wiki/%D0%A4%D0%B0%D0%B9%D0%BB:Sofia_Metro-Map.png - we can expect next year this station to be opened.