Are low ridges, typically made of coral, that rise to or near the ocean's surface.

Reefs are low ridges, typically made of coral, that rise to or near the ocean's surface.

Reefs are low ridges that rise to or lie near the ocean’s surface. Reefs usually lie at a length of 80 meters or less under low water. They are usually composed of coral, rocks or sands. Some reefs are formed from abiotic processes such as wave erosion, sand deposition while some reefs (such as coral reefs) are formed from biotic processes. An example of a reef is the Great Barrier Reef in Australia. This reef is the largest and the most popular reef on earth.

what events in mexico's history might explain why the country continues to have difficulty improving social justice and economic opportunities for all people?