I bought this because I was out of the country when it went on general release, and 2 weeks later I couldn't find a cinema within 100 miles that was still showing it, never mind the 3D version.
I'm a caver. I've caved all over the world (though sadly not in PNG, yet...). I've even been on a couple of expeditions.
What did this film get right? The location, for a start. Thick limestone + lots of water = big caves as several expediations to Borneo & China have shown in recent years. PNG was as good a place as any to base the story. Do you really get sinkholes as big/deep as that in the film? Yes, you do. 500 m deep and maybe half that across would not be uncommon. You wouldn't ordinarily parachute into sinkholes - the proximity of the rock walls and uncertain air currents would make that extraordinarily risky (though there are people that have base jumped into las Golondrinas in Mexico). Abseiling (rapelling for North American readers) on ropes is the usual method as was shown. The gear the team were wearing was pretty authentic, as was the gear they used. Modern rebreather diving technology is extraordinary; the world record for a cave dive was set last year in Pozo Azul in Spain: 8.8 km and 50 hours in water of 11C. So the basis for the film was pretty sound too. The 'passion' for caving shown by the lead explorer was also authentic. The lure of virgin discovery is extraordinarily strong.
So where did the film go wrong? Well, I've yet to hear of a caving expedition with anything like the level of resources portrayed in the film! Caving expeditions - even those on which National Geographic participates - are generally run on a shoestring. And as for running electric power down a few km of cave passge to the dive base...
Overall the film is a bit of an underground Posideon Aventure. The storyline is a bit predictable, the acting is a bit wooden, and the gory ending is unnecessary (in my view). But if you've never been down a cave, sit back and admire the majesty of the underground environment. There are bits that are CGI for sure, but there is also a lot that is real. Real caves are remarkably hostile places to take simple cameras (damp, grit, cold,...), never mind cinematography equipment. And filming cave diving is almost as logistically complex as sending a man to the moon. In fact, more human beings have been to the moon than have penetrated some of the deepest caves on planet Earth!