Coral often lives in a symbiotic relationship by serving as the foundation upon which algae live and thrive. Algae provide coral polyps with vital oxygen and carbohydrates, a byproduct of photosynthesis. ; coral, in return, releases carbon dioxide, ammonia and other nutrients needed by algae. The symbiosis also speeds up coral skeletal growth, rapid enough to build reefs faster than waves can knock them down; in this balance, coral, together with pink coralline algae, binds together a reef growing as fast as one of these volcanic islands sinks. No one knows the depth of Palmyra's coral reef; at 60-70 million years old, it could be a mile thick.