Than Nam Lot Yai Cave

The Sawi River which flows through Khao Thalu runs under the mountain by way of this cave. This is the longest cave in Chumphon at 1095m.
The 'Caves & Caving in Thailand' website has referenced this as Cave No.  CP0013.

Sink Entrance

Location: https://maps.app.goo.gl/kMftQF6mqTddPnxv9

Take Highway 41 from Chumphon town for some 50km till you reach the Khao Pip intersection (a Lotus's store is on the right just before the intersection). Turn right at the intersection onto road No. 4139 and travel 25km to the village of Khao Thalu. In the village turn right at the crossroads and travel about 100 meters taking the first road to the right. Follow this concrete road about 500m to the area of the cave. It is sign posted all the way from Highway 41.

The sink entrance (where the river enters the cave) does not provide easy access into the cave and therefore better to go to where the river exits.

Resurgence Entrance

Location: https://maps.app.goo.gl/hH4du5UWvD18dX7W7

Where the river exits the cave is a lot harder to find as there are no road signs. Took me two efforts. Same route from Chumphon town but only travel some 19km along road No. 4139. Turn right at the crossroads.

Follow the road at the base of the mountain for 5km, turning left at the T junction. The cave entrance is 500m from this junction with the entrance clearly marked.

Here you will find steps that take you down into the cave. 

Depending on the water level it is only really possible to explore the first 50m.

Chawin Srabour has done a rather good photo sphere of this part of the cave - Link

Care should be taken as it can be slippery in places. Bring a torch as there are no lights installed.

Not a very interesting cave, was lacking formations. But then we were only able to explore some 5% of the cave due to the water level. During the dry season we understand that the full length of the cave is accessible but best this is done with an organised group.