Read the following article and answer the question. Article: Imagine living in a city made of glass. No, this isn't a fairy tale. If you could grab your diving gear and swim down 650 feet into the Pacific Ocean off the coast of Washington State, you would witness the secret world of glass reefs. The reef you'd be looking at is made up of glass sponges . But how can animals be made of glass? Well, glass is formed from a substance called silica. The sponges use the silica found in ocean waters to build glass structures that will give them shape and support. Be careful! Some of the fragile creatures are up to 200 years old. When sponges die, new ones grow on top of the pile of old ones. Over centuries, a massive and complex reef takes shape. Some sponges look like wrinkled trumpets, while others look like overgrown cauliflower or mushrooms. Dr. Paul Johnson, who discovered the Washington reef in 2007, also found other surprises such as bubbles of methane gas flowing out of the seafloor nearby. The methane feeds bacteria, and the bacteria feed the glass sponges. "It's a new ecosystem we know nothing about," said Dr. Johnson. The reef of yellow and orange glass sponges is crowded with crabs, shrimp, starfish, worms, snails, and rockfish. The glass reef is also a nursery for the babies of many of these creatures and was called a "kindergarten" by scientists. Many animals that live in the reef hang around for a long time, just like the sponges. Rockfish, for example, live for more than 100 years. Scientists are just beginning to study all the species that call the reef home. The Washington coast isn't the only place where a living glass reef has been found. The first was discovered in Hecate Strait off the coast of British Columbia in 1991. Scientists all over the world were to see it. Question: Which of the following is the best title for the passage? Answer:
Glass "cities" under the sea