Cong Do (Cống Đỏ) or “Red Drainage” in English Island is located in the southeast of Halong Bay, about 25 kilometers from Bai Chay tourist wharf. It is part of the core area of Halong Bay, which is a UNESCO World Natural Heritage Site.

The island covers an area of 23.36 square kilometers and is home to a 172-meter hill covered in evergreen tropical forest. The lagoons around the island are also home to a variety of marine life, including shrimp, crab, fish, squid, and aquatic plants.

In particular, the southwestern coast of the island is home to a pristine coral reef that is 700 meters long and 300 meters wide. The coral reef is home to many rare species of coral, including red coral. It is also a habitat for seahorses and many other fish. The coral reef is in good condition.

Cong Do Island is a popular tourist destination. Visitors can go trekking up the hill to enjoy panoramic views of Halong Bay. The island is also a great place for kayaking and scuba diving.

In the southeast of Cong Do Island, archaeologists have found evidence of an ancient commercial port from the Van Don region. This evidence suggests that Cong Do Island was once an important trading post.