If anyone had asked me last month why I was planning to go to Vietnam at all, this was it. Phong Nha national park has some of the largest and most beautiful cave systems in Asia and going on an underground trek had been on my bucket list ever since I had first heard of their existence. So while this was what our plan was for day 1, the tour company was able to adjust us on the next day and off we went early in the morning. While there are multiple caves in Phong Nha, we went for the 1-day Paradise Cave trek which goes 7km underground. However, as luck would have it, part of the cave had gotten flooded and so we were only able to go 4km before having to turn back. Those 4km though, for lack of a better word, blew my mind. The trek starts with an electric buggy-ride to the foot of the hill from which one has to climb a large number of steps all the way up to the mouth of the cave. The rain was incessant and the tropical rainforest clouds are low enough to create a stunning view from up top. Descending into the cave through the entrance, the whole group collectively gasped at the landscape in front of us. A chamber bigger than any cathedral with gigantic stalactites and stalagmites all well lit up with stairs and wooden boardwalks snaking through them. So far pretty standard but that is just the first 1km. Where the wooden pathway end, our group stepped out into the cave floor, lit up our helmet-lights and ducked through a small entrance. It was then that the trek truly began as we walked through one immense chamber after the rest, ducked and crawled under rocks and waded through knee-deep water - the only light being the ones from our helmets, the only sounds being our own and drops of water dripping from the cave-ceiling. Our guide talked us through the entire trip, playing musical notes on some of the hollow diagonal stalagmites (I forget what they are called), switching off our lights and yelling together to hear our voices echo for a full minute and catching drops of water falling from the ceiling with our mouths. We also saw the strangest cave-dwellers - spiders, crickets and scorpions pale as the moon and without eyes. At the end of the 4km, we stopped for lunch which some guys had been carrying in tiffin boxes throughout the trek - home-cooked vietnamese rice with fried meat and veggies. After that, in what has to be a statement none of us on that trek ever thought we would make, we kayaked/swam in an underground lake 4 km inside the earth. After a long trek back, we were dropped off at our hostels after which we had dinner, a few drinks and tucked in for the night.