Heavy water was discovered by Urey.

Explanation

**Step-by-Step Solution:**

1. **Understanding Heavy Water**: Heavy water is chemically known as deuterium oxide (D2O). It is similar to regular water (H2O) but contains deuterium, which is a heavier isotope of hydrogen.

2. **Identifying the Discoverer**: The question states that heavy water was discovered by Urey. This is a historical fact that needs to be verified.

3. **Historical Context**: Harold Urey discovered heavy water in 1934. His work involved the separation of deuterium from regular hydrogen, leading to the identification of deuterium oxide.

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