Paradise Cave—Hang Thiên Đường in Vietnamese—stretches 31 kilometers beneath the Phong Nha-Ke Bang karst, though only the first seven are developed for visitors. British cavers discovered the entrance in 2005, but local hunters had known the site for decades; formal exploration began in 2010, and the boardwalk route opened to the public in September 2011. The cave system lies within a UNESCO World Heritage Site that protects 300 catalogued caves formed over 400 million years, when tectonic shifts and monsoon rains carved voids into Paleozoic limestone. The developed section descends through chambers that reach 100 meters in height and 150 meters in width, their ceilings hung with stalactites up to 40 meters long. Floodlighting installed in 2012 reveals calcite curtains, stone pillars, and flowstone cascades that continue to grow at roughly one cubic centimeter per century. The air inside holds steady at 18-20°C year-round, a relief during Quang Binh's summer months when surface temperatures climb past 35°C. Conservation regulations introduced in 2015 cap daily visitor numbers at 1,000, a threshold rarely reached outside the Tết holiday period. Most full-day itineraries pair Paradise Cave with Phong Nha Cave, an older show cave accessible by boat along the Son River, or with Dark Cave, where zipline and mud-bath activities cater to a younger demographic. The 7km tour designation reflects the total developed length rather than the distance visitors walk; the paved wooden boardwalk covers 1.2 kilometers, ending at a limestone peninsula the lighting engineers call "the Royal Court." Beyond that terminus, adventure caving permits allow guided treks into unmapped passages, though those require separate booking and spelunking gear. The site sits 16 kilometers west of the Ho Chi Minh Road junction with Highway 20, accessible via electric buggies that ferry visitors from the ticketing pavilion to the cave mouth. The 524-step descent takes 12 to 15 minutes at a cautious pace; handrails and LED strip lighting were upgraded in 2023 after monsoon rains destabilized the original timber staircase. The conservation fee—270,000 VND per person as of 2026—includes buggy transport and contributes to trail maintenance and speleological research programs run by Hanoi University of Science.