Vietnam (in addition to much of SE Asia) is peppered with the ubiquitous yet quite remarkable karst pinnacles. There are a number of places where rivers flow through and under these outcroppings. While most of these waterways were shallow and the caves low, there are some with caves tall and rivers navigable.
Here, I got a shot of a local's boat motoring through one of these large caves.
Location: Ba Bể National Park, Vietnam
Lens used: 24-105mm f4.0 IS
From Northern Vietnam and Laos