If you want to see crazy sights, visit Son Doong, located deep in the juggles of Phong Nha Ki-Bang National Park in Vietnam.
It is the largest cave on the planet: 600 feet tall, by 300 feet wide and 2.5 miles long -- and that's just the largest chamber, not the entire cave.
You could fit an entire Chicago block inside that chamber -- with the skyscrapers! It’s so huge that it has its own weather system with clouds.
It also has its own ecosystem. Huge natural skylights, 300-feet across, provide light for the inside areas. Because the cave is lit, there is life. Inside the cave you’d see a rich subterranean jungle filled with rare plants and hanging vines winding around enormous stalactites and stalagmites. There's strange white insects and even a troop of monkeys, the only ones on earth known to make their home underground.
It isn't easy to visit. To keep the cave as pristine and natural as possible, only a handful of tourists are allowed in each year, part of a five-day trip, two nights camping out in Son Doong itself.