[he might not have been down there when it happened, but he just knows one of his crew had to be responsible. At least, he can guess. It feels like Luffy. But this is Zoro, and thus, no matter how determined he is to find the exit, he is likely the last of his crew to actually get there. He stops in the entranceway of the fortress, looking back.
Straw Hat Crew
Where are Luffy and Sanji?]
...oi, is everybody out?