The Mountain Dragon King was of the earth element itself. He relied on his great strength to dash into the earth and used his massive body to control his speed when rapidly drilling underground.
His hands were full from protecting Long Yuxue, so he could only use the sharp edges of his wings to open up the soil. His physical capabilities were far beyond human standards. Soon, they had bored dozens of meters into the ground.
It would be very difficult even for a seismograph radar to detect their presence so far down and the result would be quite blurry in any case.
However, it was not easy either to move at such depth, leave alone control his speed.