Only the scrape of our boots against rock and loose pebbles broke the silence as we moved deeper into the cave. Breathing had become an effort. The air, long stagnant from being kept in this tomb-like cave, burned our throats and left a rancid taste. Foul smelling and heavy, it seemed to stick in our lungs after each intake of breath. The miner's lamps on our helmets cut a thin slice out of the thick, black gloom around us. Everything below us, above us, and to all sides of us was black and unknown. We moved carefully, groping our way among the slimy rocks, slipping and falling into potholes and pools. The hollow darkness echoed with our shouts and curses. We never expected to see the light of day again, but we pushed on.

Q. Underline the sentence which may be considered proof for the answer to question No. 2.