How can i find corrupted lake and underground?
You can find them inside the cave past the Bridge the Troll guards, after heading there the first time, talk to the lizardess in the ruins or the witch in her hut on the swamp about the cave and do what is needed to get the item for it, after you get it, equip it before gonig back there and you can explore the underground cave, then go between the first town and the cave to unlock new spots inside it randomly, the monsters in there can lower sanity i think, but i know the 2 slimes and the raptors lowers the sanity meter.
Cilck on that cave and the town icon or any of these little dots to move around, i think it is random when you find new locations in there each time you do that, and if you wondering about unlocking a shortcut back to the human town, head there in the undergrond from that cave and you should be able to unlock it so that you can return to the underground from there instead of the cave.