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.
The sea attacks cracks in the cliff at an headland. The cracks grow larger and form a cave.
financial collapsed during the great depression in Austria
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.