You could say the Veryovkina Cave in Georgia is the biggest room in the world, if you define a room as a single continuous enclosure not impeded by any barriers or gates. It’s referred to as the Mount Everest of caves and has six points of entry once thought to be unrelated. My best friends are cave hobbyists (my body isn’t ready as they say, though to be fair neither are theirs for different reasons), seeing/capturing never before things all the time, and are probably evading the law that far below our overworld right now.