So, About the Hellmouth

[ILLYRIAN]

When the big yellow school bus was leaving Sunnydale it stopped so the characters could take a look at the hole in the ground that was Sunnydale.
Question - considering how big an area dropped into the Hellmouth how did the town stay above ground?

Then in season 1 Buffy went down to fight The Master - Hell seemed such a tiny little place.

 

[Kendar]

If the caverns are deep enough it's not that big a deal. There are caverns that you can enter (for a fee) near where I live, and they stretch about a mile (including under our house). They go under a major interstate (I-35). But the top of them is 75 feet below our house, so it would take a major event (like a Hellmouth) to cause issues.

It's also possible that the Master was only using a small part of the area. There are 4 tours for our caverns that each cover a different amount of distance, so it's possible to only see a small part. The biggest tour actually requires crawling to get to larger areas.

 

[Stake fodder]

Question - considering how big an area dropped into the Hellmouth how did the town stay above ground?

It could be that the town was kept from collapsing by supernatural powers, and once the First was defeated, boom!