After one of these periods of oblivion he began to hear a dull, steady thunder. He could no longer crawl. The bore had shrunk to a mere crack, barely large enough for him to pass through wormlike on his belly. He sensed now a deep, shuddering vibration in the rock. It grew stronger, terrifying in that enclosed space. Steam slipped wraithlike into the smothering air.
The roar and the vibration grew to an unendurable pitch. Trevor was near to strangling in the steam. He was afraid to go on, but there was no other way to go. Quite suddenly his hands went out into nothingness.
The rock at the lip of the bore must have been rotten with erosion. It gave under his weight and pitched him headfirst into a thundering rush of water that was blistering hot and going somewhere in a great hurry through the dark.
After that Trevor was not sure of anything. There was the scalding heat and the struggle to keep his head up and the terrible speed of the sub-Mercurian river racing on to its destiny. He struck rock several times, and once he held his breath for a whole eternity until the roof of the tunnel rose up again.
He was only dimly aware of a long sliding fall downward through a sudden brightness. It was much cooler. He splashed feebly, because his brain had not told his body to stop, and the water did not fight him.
His feet and hands struck solid bottom. He floundered on, and presently the water was gone. He made one attempt to rise. After that he lay still.
The great mountains leaned away from the Sun. Night came, and with it violent storm and rain. Trevor did not know it. He slept, and when he woke the savage dawn was making the high cliffs flame with white light.
Something was screaming above his head.
Aching and leaden still with exhaustion, he roused up and looked about him.