Grotto Heaven, located about 150,000 meters beneath the surface of the Earth.
There is a babbling flow of magma from underground, like tidal waves, surging and making a "whoosh" noise.
The rocky walls gently vibrate, sometimes soft, sometimes contracting.
Lu Yuan, with his eyes closed, carefully discerns and feels an overwhelming power, almost like the thundering of drums. However, this sound is inaudible to ordinary people; it is a form of Mind wave, with very low frequency, pulsing only once per hour.
Thump! Thump!
This time he truly felt a deep shock; this was no ordinary Grotto Heaven... it was the heart of a living creature!
The underground tunnel they had just traversed was, in fact, merely a...blood vessel!
That is to say, the rail train traveled thousands of kilometers along the blood vessel before entering the heart.
"This really is a corpse..."
"An Ancient Giant Demon's corpse?" he exclaimed in genuine astonishment.