Endangered underwater ecosystem

REEF

Think of a vibrant, biodiverse structure built by tiny marine architects, often called the "rainforest of the sea" but found beneath the waves. Coral plays a key role here.

The clue "Endangered underwater ecosystem" points to "reef" because coral reefs are vital marine habitats currently at risk from climate change and pollution. The word "reef" directly names these structures, which support diverse sea life but face threats like bleaching and overfishing. No wordplay is involved—it’s a straightforward definition matching the answer.