Phong Nha-Ke Bang National Part of Quang Binh - Vietnam is the largest Karst area of the world, mroe than 300 caves have been discovered and thousand unknown caves.
Son Doong Cave: World's largest cave: http://oxalis.com.vn/GL/en/aboutus/217/
Hang En cave: Featured on NatGeo as best cave: http://oxalis.com.vn/GL/en/aboutus/159/
Tu Lan cave system:  http://oxalis.com.vn/GL/en/aboutus/217/

Welcome cavers to explore the area. All info available on Lonely Planet Guide Book 2012
Cheers