I've never heard of this place before, but damn it looks impressive: Son Doong Cave
, the world's largest cave.
The Son Doong Cave in Vietnam is the biggest cave in the world. It's over 5.5 miles long, has a jungle and river, and could fit a 40-story skyscraper within its walls.
visiting this is going on my bucket list. the pics are awe inspiring!
there's more pics at this site