What was it like inside? Dark and cold and damp? Were the rocks / ground supported? How deep could you go in?

If this is the cave I’m thinking of, it’s huge.

kagis

I don’t know if there is a “Phong Nha cave”, but the one I’m thinking of is in Phong Nha-Kẻ Bàng National Park, so I’m guessing that that’s it.

https://en.wikipedia.org/wiki/Hang_Sơn_Đoòng

Sơn Đoòng cave (Vietnamese: hang Sơn Đoòng, IPA: [haːŋ1 ʂɤːn1 ɗɔ̤ŋ2]), in Phong Nha-Kẻ Bàng National Park, Quảng Trị Province, Vietnam, is the world’s largest natural cave.[1]

I think that the above image is from a sinkhole where the roof opens up partway through it.

There’s some YouTube video I saw a while back that did flybys of 3D models.