### Theory:

‘It’s easy to walk here,’ he said, but the distances are long. Selenopolis occupies more than a hundred square kilometres. Let’s ride for a bit’.

A moving pavement covered with green plastic strips rushed them through echoing tunnels and spacious caves with walls sparkling in the light of the quartz lamps. The elevator delivered them to the surface. She looked in silence at the enormous lily-shaped tent over a crater that was at least a kilometre in diameter.
Explanation:

Since the astronaut sensed something was amiss and was intent on keeping his journey toward another galaxy a secret, he attempted to change the subject during the conversation. He agreed with his mother's opinion that walking on the moon was quite simple. He then told her that Selenopolis is almost $$100$$ square kilometres in size. Then they rode on a rolling pavement with green plastic strips covering it.

Moving pavement

The mother and son travelled through tunnels and caves from the moon's surface. The walls of those caves sparkled in the light of the quartz lamps. A quartz lamp is an electric lamp in which the envelope is made of quartz, which allows ultraviolet light to pass through it.

The tunnels echoed their voices because the place appeared to be uninhabited and the caves were large. Their journey came to an end once they returned to the moon's surface via the underground channel after some time. She stood there calmly staring at the massive lily-shaped tent situated above a crater with a circumference of at least a kilometre.
Reference:
State Council of Educational Research and Training (2018). Term-3 English Standard-9. Mother’s Voice - Vasil Berezhnoy (pp. 170-173). Published by the Tamil Nadu Textbook and Educational Services Corporation.