Prior to arriving in Cat Ba, I'd never heard of Phong Nha before. When I got to my hostel, though, I looked at the list of options by bus and Phong Nha happened to be between Cat Ba and the next stop on my list, Hoi An. A quick Google image search gave me lots of gorgeous pictures of caves and jungle, so I decided to book the ticket. I would take a bus leaving at noon, first to a town called Ninh Binh, about 6 hours from Cat Ba, and then overnight to Phong Nha, arriving around 4am. Everyone I met in Cat Ba that had been to Phong Nha recommended the same hostel: Easy Tiger. It wasn't on the usual booking websites I used, so I had to email them to make my reservation, but they luckily had 3 nights available on the short notice I gave them.