Behind the cut you'll find some information about who was filming today, where the scene seemed to be set, and what monsters are going to make an appearance.
First up, the official BBC blog confirmed the return of the Zygons!
From the blog post:
Aside from flashbacks these malevolent shape-shifters have only appeared once before, threatening Earth in the Fourth Doctor adventure, Terror of the Zygons. Despite the fact that their solitary outing was over thirty years ago they remain a popular old enemy and in the 2010 episode, The Pandorica Opens, they were amongst the massed aliens said to be gathering above Stonehenge. Later, in The Power of Three, we learnt that Zygons had one of their ships under the Savoy Hotel in 1890 and apparently replaced half the staff with imposters! But this time they really are back…
Pictures of filming were snapped and in the following pictures you'll see evidence of David and Billie on set, the medieval England appearing setting, the RTD era Tardis, as well as a Zygon wandering around.
source 1
source 2
Speculate away! What could this mean for the plot of the special?