On the 16.11.2013 some 100 tonnes of columnar basalt collapsed in Hálsanefshellir cave. Fortunately, nobody was in the cave at the time. Here is the story on the collapse in Hálsanefshellir. It is in Icelandic, but there is a photo of the collapse so you can see how dangerous it would have been to be inside the cave at that time. The collapse was from the east side of the cave.

https://guidetoiceland.is/connect-with-locals/regina/extremely-dangerous-waves-by-reynisfjara-and-kirkjufjara-in-south-iceland