Phonf Nha-Ke Bang is a national park and UNESCO WHS in north-central Vietnam. The core zone of this national park covers 857.54 km2. The park was created to protect one of thr world's who largest karst regions with 300 caves and grottoes and also protects the ecosystem of limestone forest of the Annamite Range region in north-central coast of Vietnam.
You can read here about a longst cave in the world, found in 2009.
Thank you Johnson!