Hang Én, the world's third-largest cave, offers an unparalleled multi-day adventure accessible only through licensed guided expeditions, typically lasting 2 days and 1 night. Due to the challenging trek through dense jungle and river crossings, excellent physical fitness is required, and solo visits are strictly prohibited for safety and conservation. The cave is generally open from late January/early February to late August/early September, closing during the monsoon season due to dangerous water levels. Costs are significant, ranging from $300-$500+ USD per person, which typically covers permits, expert guides, porters, equipment, and meals, necessitating booking several months in advance. Visitors can expect to witness immense chambers, unique rock formations, and the namesake swallows, making for an unforgettable, though demanding, wilderness experience.