Shen Yue was playing dumb.
Obviously, he knew that there wasn't an immortal treasure in the hole he was looking into, he was just observing the Three Kings' reactions towards it. Their body language, their reactions to the situation, could reveal their habits, even their inner selves.
Then he would decide his next move based on their responses.
King of the Sea, not knowing the depth of Shen Yue's thoughts or able to see the bottom of the hole either, knew to throw a rock into it.
The "King of Knowing Better" immediately bowed to the ground to listen.
Shen Yue started to see the "King of Knowing Better" in a new light — this was a very ancient skill. In olden times, this technique was often used for spying on the enemy. Clever generals defending the city would bury large urns in the ground, using them to listen for the sound of the enemy digging tunnels.
The "King of Knowing Better" seems to have a pair of decent ears.