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Lihou Reef and Cays is the largest atoll by total size in the Coral Sea. It is a part of the Coral Sea Islands Territory. The atoll is U-shaped, with an opening in the southeast. The size of the lagoon is about 100 by 30 km², with an area of 2500 km², which makes it the atoll one of the largest in the world. Nevertheless, the total land area of the islets and cays is little more than one square kilometer. The lagoon is up to 60 m deep.

Lihou Reef and Cays were discovered by Lieutenant John Lihou, R.N., on 23 February 1823. Lihou, then Master of the merchant ship Zenobia, was on passage from Manila to South America. After a hazardous passage through the reef the ship departed the Great Barrier Reef via an opening near Murray Islands. On 23 February, Lihou Reefs were sighted. Source: Hydrographic Service - Royal Australian Navy

The atoll and the waters around were declared, on August 16, 1982, the Lihou Reef National Nature Reserve, with a total area of 8440 km², to protect wildlife.

There is a total of 18 islets and cays, 9 each on the Northern Rim and on the Southern Rim. They range in size from 5,000 to 168,000 m².

Total land area 560,000 m² The islets on the Northern Rim are, from southwest to northeast (clockwise):

1. Juliette Cay (78,000 m²)
2. Kathy Cay (40,000 m²)
3. Lorna Cay (168,000 m²)
4. Little Margaret Cay (10,000 m²)
5. Margaret Cay (30,000 m²)
6. Turtle Islet (31,000 m²)
7. Middle Cay (78,000 m²)
8. Observatory Cay (88,000 m²)
9. Licklick Cay (40,000 m²)

Only Lorna Cay and Turtle Islet are vegetated. Turtle Islet reaches a height of 6 m, and Observatory Cay 2 m. There is an automatic weather station on Turtle Islet.

Total land area 470,000 m² The islets on the Southern Rim are, from northeast to southwest (clockwise):

1. Anne Cay, "No 1 Cay" (112,000 m²)
2. Betty Cay, "No 2 Cay" (37,000 m²)
3. Carol Cay, "No 3 Cay" (5,000 m²)
4. Dianna Cay, "No 4 Cay" (16,000 m²)
5. Fanny Cay, "No 5 Cay" (7,000 m²)
6. Edna Cay, "No 6 Cay" (73,000 m²)
7. Helen Cay, "No 7 Cay" (10,000 m²)
8. Georgina Cay, "No 8 Cay" (105,000 m²)
9. Nellie Cay, "No 9 Cay" (104,000 m²)

Only the largest three of the Southern Rim, Anne Cay, Georgina Cay and Nellie Cay, are vegetated. Just Northeast of Anne Cay is the Herald Passage into the lagoon.

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