What are coral reefs? Which is sea is called coral sea?

Explanation

### Step-by-Step Solution:

1. **Definition of Coral Reefs**: Coral reefs are underwater structures made primarily of calcium carbonate, which is secreted by corals, a type of marine invertebrate belonging to the phylum Cnidaria.

2. **Formation of Coral Reefs**: Coral reefs are formed when coral polyps attach themselves to a hard substrate, such as rocks, on the ocean floor. Over time, these polyps secrete calcium carbonate, creating a skeleton that builds up and forms the reef structure.

3. **Characteristics of Coral Reefs**: Coral reefs are known for their vibrant colors and diverse ecosystems. They provide habitat for numerous marine species and are often referred to as the "rainforests of the sea" due to their biodiversity.

...