The Manacota Underground river is located in Marag Valley, Apayao.

Upon going to the underground river, you need to pass through the Manacota River itself for like 30-mins to 1 hr. 

And of course you need to ride a boat upon entering the cave because the water is deep. It's really cold inside the cave, and you really need a lot of flashlights for you to be able to see it's beauty inside. 

And as you can see the water is really clean. And because it's water is continuously running, you might get taken away by it's currents. So always be careful!