A shake-hole, sink-hole or doline. The collapse may also form an entrance to the cave below, if the slumped material has not filled the passage.
No, first it was a city, then a Republic, then a Empire, and then it collasped.
caves
primary
The plural form is CAVES.
When the rock above a cave collapses, it can form a sinkhole if the collapse creates a depression in the ground. This can sometimes lead to the formation of a sinkhole cave system underground.
The word cave can be a noun and a verb. The noun form is a large cavity formed underground. The verb form means to surrender.
Cave painting
Cave drawings.
financial collapsed during the great depression in Austria
The sea attacks cracks in the cliff at an headland. The cracks grow larger and form a cave.
Waves keep hitting the side of a cliff and eventually erode enough rock to form a cave
That's extremely variable and specific to each cave, but you have to think in tens or hundreds of thousands of years.