Until Ivan Trifanof, no one had flown a hot air balloon into a cave. Ivan did just that, flying to the bottom of Mamet Cave in Croatia.
Ivan used a smaller than usual balloon with custom gas tanks fitted, rather than the traditional basket. This gave him added maneuverability and more clearance from the sides of the cave.
This is some stunning piloting, imagine the planning that went into this mission!
Thanks to AdventuresCroatia for this video.