This was a volcanic crater, with the topmost rim only about a hundred meters in diameter, yet the space within the belly of the mountain was nearly a kilometer across. Looking down from above, only the center was visible, while the surroundings of the area were completely out of sight from above.
Moreover, given the height, even if Shui Qingyue were to scream at the top of her lungs, she wouldn't be heard from above. Moreover, the mountain outside is quite steep; ordinary people could not climb to the top. Even if those people were tracking them, they likely wouldn't think that Song Xiaodong could climb to the top and then descend.
Therefore, this place was absolutely very safe.
Inside this part of the mountain, the temperature was just as cold; it wasn't much warmer even though it was a volcanic crater. Only the hot spring in the very center was the warmest spot.