Locat­ed in the north­ern part of cen­tral Viet­nam, Phong Nha Cave is the sec­ond largest cave in the coun­try and a UNESCO World Her­itage Site. Phong Nha is sur­round­ed by Phong Nha Ke Bang Nation­al Park. The park has near­ly 300 caves, though only a frac­tion have been sur­veyed in any detail. Despite the exis­tence of such a vast under­ground net­work, tourists are per­mit­ted only in the first mile or so of Phong Nha’s tun­nels.