Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions I'm standing on a sidewalk in the early morning. The great avenues of Paris are silent, and the shops are closed. Suddenly, a couple appeared from a hole in the sidewalk. Mud covers their boots. They place the iron cover over the hole and run down the street, smiling. The couple had been exploring the tunnels beneath the city. When Romans occupied Paris, they ...

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions I'm standing on a sidewalk in the early morning. The great avenues of Paris are silent, and the shops are closed. Suddenly, a couple appeared from a hole in the sidewalk. Mud covers their boots. They place the iron cover over the hole and run down the street, smiling. The couple had been exploring the tunnels beneath the city. When Romans occupied Paris, they cut down stone from deep within the earth to build their city. Later, the French used more stone to construct the Notre Dame Cathedral. This left huge underground tunnels upon which part of the city now. Once used for growing mushrooms and as a hiding place, today they are mostly forgotten, except by "cataphiles" - those who love to go down into the tunnels below Paris, even though it is actually not permitted. In the 70s and 80s, it is easier to enter the tunnels because there were many more open entrances through forgotten doorways and into catacombs - rooms filled with bones. The bones had been moved into the tunnels to solve the problem crowded cemeteries. By the end of the 1980s, most of the entrances were shut and police regularly walked the tunnels. However, there are still cataphiles, like the couple I saw that morning, and for those who dare, the underground is an exciting place to meet, party, perform for each other or create art. My own experience began beneath the old Paris opera house, where sewer workers showed me a 55-meter-long underground pond, a pond that actually had fish in it! Later, at France's national bank, officials guided me below to an amazing room filled with 2,600 tons of gold. As cataphiles are the best guides, I then asked one of the cataphiles to give me a tour. Descending into the underground through a secret entrance beneath a bridge, we walked for hours through catacombs, and galleries of huge, bright paintings. The cataphile told me "Many people come down here to party, some to paint. Some people to destroy or to create or to explore. We do what we want. We don't have rules." (Adapted from Reading Explorer 2, Paul Macintyre&David Bohlke, 2015)

What is the reading mainly about?