AceShowbiz -

The Castle Black on "Game of Thrones" Season 6 was almost destroyed by rockslide during filming. According to EW, The Castle Black scenes are filmed in Northern Ireland's Magheramorne Quarry, where a practical and functional castle set rests at the bottom of a rockface.

The rock wall is then dressed as "ice" and extended with special effects to look like The Wall. Many scenes shot in the quarry are actually filmed during an extremely cold winter downpour, which viewers don't see on TV as rain typically doesn't show up on camera.

The heavy rain apparently disrupted the quarry's "seismic profile," as experts later explained to the filming team. Showrunner Dan Weiss revealed, "Once people saw the small pebbles start to come down that turned into slightly larger pebbles, everybody made the group decision to immediately step away from the set." Owen Teale added, "A piece of rock the size of a London townhouse just fell."

Nobody was hurt when the rockslide happened early during the production year, but it caused an enormous challenge for executive producer Bernie Caulfield and producer Chris Newman to map the shooting schedule.

Dan explained, "Bernie and Chris already have hardest scheduling job in the film and TV industry, and they had this thrown into the works too, which caused them to drastically reschedule the whole season on the fly. But they did it, and we got everything we needed in that quarry after it was seismically shored up and netted and proper protection measures were taken. It actually worked out for the best - some of the stuff we were shooting in that location benefited from the additional prep time the rockslide gave us."

"Game of Thrones" Season 6 premieres on HBO on April 24.