Accessible only by boat, it is the journey on the river in a traditional longtail boat and the visit of the caves with incredible stalactites and stalagmites that make this an interesting visit. But in general, Khao Khanab Nam is part of a whole boat tour in Krabi that also goes through the mangrove tunnel and Koh Klang Island (see below, the tour I recommend).
You can climb in the main cave to an opening that takes you outside to the top. A large number of human skeletons have been found here. While their origin is uncertain, it is believed they were people who settled down here but perished in a flood.
To go visit Khao Khanab Nam, you can also book one of the tours above.