Coral reefs are a landform created by living organisms, specifically invertebrates called corals.

Here's how it works:

* Coral polyps: These tiny animals build hard, calcium carbonate skeletons around themselves.

* Colony building: Coral polyps live in colonies, and over time, their skeletons accumulate, creating large structures.

* Reef formation: These structures grow and develop into different types of reefs: fringing reefs, barrier reefs, and atolls.

So, while coral reefs are not technically landforms in the same way a mountain is, they are massive, underwater structures created by the accumulated skeletons of millions of tiny invertebrate animals.