⚡ Free Adventure
Phong Nha National Park is home to Son Doong, the world's largest known cave passage. Son Doong could fit a 40-story skyscraper inside. The Dark Cave experience is essentially a theme park built inside a living geological wonder, and it costs $15.
Best time: Dry season (February–August)
Risky activity: go with a guide or experienced locals, know your limits, and check your insurance.
