Imagine if you could put a drone inside a 5.5 miles long cave that is as tall as a 40-story skyscraper. Plus it has a river flowing through it. Hang Sơn Đoòng (or mountain river cave) is the largest cave known to mankind and the mere fact that they allowed a crew along with a drone to shoot inside is not trivial at all.
Photographer and cave explorer Ryan Deboodt spent some time in the cave along with a Canon 6D, a 16-35mm f4 lens, a DJI Phantom 2 and a GoPro Hero 4 Black edition to give a glorious view of the cave. It is breath taking.
It is amazing that footage like this can be taken with less than $4K worth of gear.