This cave located at a dozen kilometers from Phatthalung in the district of Khao Chaison, area whith several interesting visits even if its floating market closed for lack of customers, can only be visited by boat with a local guide because has an underground river. Inside you will see interesting rock formations and stalactites some of them glitter and bats of course. You will feel a little bit like an explorer having to lie down completely on the boat sometimes to be able to pass. Otherwise, there is also the cave of the tiger and its lights turning from blue to red which overhangs the immersed part.
I recommend you to cumulate the visit of the Nam Yen cave with the Tham Phra, cave of the Buddha and Khao Chaison Hot Springs at the end of your program (see page about visits to do in Phatthalung) both places are less than one kilometer away. I also found a nice corner to eat almost sitting in the river, that I indicate on the page dedicated to the cave of the Buddha. I really encourage you to go to this friendly district that most guide books about Thailand do not mention and whose attractions are under threat for lack of tourists.