This cave is so large it has it's own beach

here, vietnam http://oxalis.com.vn/cave/hang-en-cave/

Thank you! I’ve got a new place for my bucket list

my bucket list reads like an encyclopedia

Seriously. The Postojna cave in Slovenia is so big, it has a 5.3 km mini railway through it, and a central hall with a mini train station. There's also a concert hall that seats 10,000.