Below is a video that I found interesting — it concerns the much larger number of sinkholes we've had over the past 10 years and one theory as to what causes them and why the number is increasing. It takes 13 minutes to play — so open it when you have a bit of time. In a way, this theory seems far-fetched. In another way, it makes a sort of sense.
Even if you're not into the new geology theory presented here, the photography is stunning and shocking:
https://www.youtube.com/embed/TLUgEXI9RYI?rel=0