Answer:
Coral reefs and sandbars create lagoons.
Step-by-step explanation:
"The combination of coral growth and water creates a lagoon. It may take as long as 300,000 years for an atoll formation to occur. Coastal lagoons form along gently sloping coasts. They are generally shallower than atoll lagoons and tend to be separated from the ocean by an island, reef, or sand bank." I got my information from google :)
Hope this helps :))))