Final answer:
Ziggurats were the massive Sumerian temples built as religious structures in ancient Mesopotamia.
Step-by-step explanation:
The massive Sumerian temples were called Ziggurats. These were stepped pyramids made of mud bricks, built as religious structures by the ancient Sumerians in Mesopotamia. Ziggurats had multiple levels and were often dedicated to specific gods or used as centers of worship and administration.
Learn more about Sumerian temples