You set off again. Within 30 minutes or so the lighted areas give way to younger, darker passages. The rocks are jagged and rough. Recent (relatively speaking) rockfalls and sinkholes are evident, making the going treacherous.
You begin to pick your way through the boulder strewn caverns. It's a bit tedious and slow going but you make progress. Orsik's light spell is able to keep almost the entire party in at least dim light which helps even those who have dark vision get a better view. As you move along you feel the stone around you rumble on occasion. Pebbles and bits of debris fall and skitter over the rocks around you. It's pretty disconcerting.
After a couple of hours of this the anxiety it creates is starting to weigh on you. Then it happens. First a rumble. Then debris. Then bigger pieces. The ground starts to shake. The epicenter seems to be behind you. Someone near the back of the party screams for everyone to run and you do. But there is no escaping. Fist and head sized boulders then even larger ones start dropping around you.
OOC:
Make a DC 12 Dex save. I'll give you advantage due to the additional light from Orsik.