We managed to receive the second Lung Moss around the 4th floor of the cave. We need 4 and we already got 2 so we are almost done. However, it seems our luck seems to have been taken away after our last encounter with the Deep Sea Curse Snake. We didn't meet any of those snakes again and finding the Lung Moss is now too hard.
Currently, we are on the 10th floor of the cave. The cave has 15 floors in total and we are already down to 5 floors that haven't been explored. Ah yes, we also managed to obtain the second broken piece of the artifact which means the other artifact piece might be further below the underground cave.