By now, all these other reviews and photos make clear the surreal experience and wonder when visiting Son Doong. There's nothing like it. I likewise had an excellent experience with the coordination and professionalism of Oxalis. So for the review, I'll just focus on a few things I wish I knew before going, which will hopefully help others make a good decision.
1) For those who are reasonably fit, and perhaps even those who do not exercise a few times a week, this is not that difficult a trek in terms of cardio. Some of the other posters here make it sound like hiking Son Doong is akin to Army basic training or something. I assure you it is not. Look at Oxalis' actual physical checklist. You'll see things such as "Can you climb six flights of stairs without stopping or getting out of breath?" or "Can you easily climb over a wall the height of your waist?" I found that relatively modest amount of fitness to be adequate for our good-weather trip (muddy but no rain. low 90s in daytime but cool at night and cool in cave). I bet doing it in, say, August is more difficult.
The hike is now 4 days/3 nights and perhaps the old 5 and 6-day versions were also harder. The new challenge on the 4-day version is climbing the "Great Wall of Vietnam." We did find this to be the most difficult segment but you're harnessed up, guides are helping to pull somewhat on the difficult parts and it's over really quickly. It's not an upper body strength thing. You're leaning back and still using your legs (I guess you use your arms a little bit to pull but it's less effort than 10 pull ups).
Some perspective on us...mid-30s. Longest recent one-day trek we had done before this was 20km. Longest recent multi-day trek we had done was Inca Trail. Not overweight but not hitting the gym every day either. We did not find the most difficult day on Son Doong to come close to the effort of that one-day hike or to the most difficult day on the Inca Trail. There were a couple older less fit (and a bunch of older and younger, more fit) people than us on the trail. Everyone seemed to make it through comfortably. It's not a race and when you get tired or winded, you stop.
Our guides suggested that people turning back on the first day was extremely rare and that they mostly self selected after things like getting really bad blisters or injuries. Get the shoes they suggest, wool socks, treat blisters quickly, and rely on their training advice (which is easy stuff, like doing 10 km day hikes a handful of times before coming) and this will be no problem.
2) The more-unique physical challenge is the type of terrain, balance, and mental focus required. The cardio is easy, but this is one hike where it feels like you could break an ankle on almost every step. Extremely sharp rocks that you sometimes must balance on. Everything is slippery. Always searching for handholds. Often getting on your knees or butt to keep from falling over. And doing that for five hours or whatever straight a day.
That was the challenging part for us. It was just mentally exhausting after a while to always be worried about making the right step. The guides and safety guides (probably more than one for every two tourists) are always there to help.
3) The right equipment and packing makes a huge difference. Get the shoes they recommend and the socks they recommend. So glad we did. And for what you actually pack, carry the absolute minimum yourself. We had $10 backpacks from amazon that fit nothing but our helmet, camera, tripod and Oxalis-provided water bottle. That's it and we never felt we needed something during the day. Porters carry the rest. Some people carried more serious, large bulky bags and I never observed what benefit came from doing that. Get quick-dry stuff for clothing and plan to wear most of it for multiple days of the four day hike. Also, they claim there is a "dry" day on day three, but even if you're not actually knee deep in water, it's still really muddy so we didn't see a benefit to another pair of shoes.
4) If you have trouble on the rocky stuff, find one of the guides who's especially attentive and stick behind that guy. To be clear, all of the guides are extremely helpful when you ask and everyone knows where guests have trouble and are attentive to those times. But some of the safety guides were definitely more the type to turn around every couple steps and just make sure you were getting it OK. During those times, I found the hiking much easier than when I was "keeping up" and trying to figure it out on my own.
5) It's definitely worth the money, but if you're really money constrained, there are better things to spend it on. $3,000 per person is a big deal to us. But ultimately we won't miss it in six months, while we'll be keeping this experience for forever. However, if I were in my early 20s and that $3,000 could be used for this or, say, another two months in Vietnam, I would go for the latter. The four days are unique, amazing and one-in-a-lifetime but not worth going into debt or sacrificing all those other once-in-a-lifetime experiences you can have with $3k when you're young and poor.
Anyway, a lot more words than I intended, but hope it helps. Don't let the physical stuff psych you out and go!
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