Hung Thoong Cave System

Hung Thoong is a system of four connected caves, and it is the wettest trip sold in Phong Nha: the centrepiece is a swim of roughly one kilometre through the flooded passage of Tron cave, and water shapes almost every hour of the three-day itinerary. That makes the decision simple in both directions. If you are a confident swimmer who wants caves measured in swim time rather than boardwalk length, this is the best trip in the park for you. If you are a nervous swimmer, be honest with yourself and book something else, because there is no dry version of this route and a kilometre of water in the dark is a bad place to discover your limits. It runs as a three-day, two-night expedition with Jungle Boss at 12,000,000 VND, minimum age 16.

The Tron cave swim is the reason people book. The passage floods wall to wall, so you swim rather than wade, roughly a kilometre through still, startlingly blue water with a headlamp beam and the group's lights for company. Life jackets are provided and safety staff swim with you, so the skill required is comfort rather than speed, but it is sustained time in deep water inside a dark cave, and that combination is exactly what it sounds like.

The rest of the system keeps the pace. A 60 metre zipline crosses the sinkhole at Nightmare Sharp cave, a 25 metre rope descent takes you down inside, and the route also threads Hung and Thung caves. Between the caves sits the Floating Lake, a lake in a forest basin whose level rises and falls with the water table, sharply higher in the wet months and shrunken back in the dry, so the lake you camp near is never quite the same lake twice. The system only opened to visitors in January 2023 after the usual years of survey and licensing, and it remains one of the quieter corners of Phong Nha-Ke Bang National Park, which for a trip built on still water and silence is worth something.

Who else should pick something different: anyone under 16, since the age limit is firm, and anyone whose main goal is a single giant chamber, since that is Hang Pygmy's territory at 7,900,000 VND. The three-day Tiger Cave series at 12,500,000 VND is the closest-priced alternative and is drier, bigger on cave scale, and camps at the Kong Collapse sinkhole. Hung Thoong wins on water, novelty and quiet. The tours run with Jungle Boss, the licence holder for the system, and the discount code JBR5 takes 5% off at checkout.

Season matters more here than anywhere, because the trip is made of water levels. The dry window is February to August with March and April the sweet spot, and from September to November flooding closes the expedition caves entirely; a route with a kilometre of flooded passage is the last place to gamble on shoulder-season weather. Cave interiors hold around 23C year round, and after hours in the water you will want the warm layer the packing list tells you to bring.

The trip runs from the Phong Nha area, reached the standard way: Dong Hoi, about 45 km out, has the airport and the mainline station, about 3 hours by train from Hue, 4 to 5 from Da Nang and about 10 from Hanoi on the overnight sleeper. From Dong Hoi, a taxi takes 45 to 90 minutes to Phong Nha town, or the local bus about 90 minutes for around 35,000 VND. Fix the tour dates first and let the trains follow.

Don't miss

• Four connected caves: Tron, Nightmare Sharp, Hung and Thung
• A swim of roughly 1 km through the flooded passage of Tron cave
• 60 metre zipline across the sinkhole at Nightmare Sharp
• 25 metre rope descent
• The Floating Lake, rising and falling with the water table
• Opened to visitors in January 2023, still one of the park's quietest systems
• Three days and two nights with Jungle Boss at 12,000,000 VND, minimum age 16

Good for

Getting there

Know before you go

• Roughly a kilometre of continuous swimming through a flooded cave passage. Life jackets are provided and safety staff swim with the group, but this is a lot of time in deep water.
• If you are a nervous swimmer, choose a different trip. There is no dry route through Tron cave and no way to skip the swim.
• Minimum age 16, three days with two nights camped.
• The 60 metre zipline and 25 metre rope descent are rigged and controlled by staff; no prior experience is needed.
• Opened in January 2023, so it remains one of the quieter systems in the park.
• Waterproof everything: dry bags for anything you cannot afford to soak, and a genuinely waterproof camera setup if you want photographs.
• Cave interiors hold around 23C, which feels cold after long swims. Bring the warm layer the packing list specifies.
• Book dry-season dates only. Water levels rule this route, and September to November is off the table.