The Phoenix Project: A New World
 A Speculative Mystery Novel

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In my story, the detectives are introduced to an underground world.

Doing research, I found a similar place.

Son Doong Cave Vietnam:

World's Largest Cave that has Forest and a River

New Cave Found  use this link to read entire article.

Son Doong Cave in Vietnam, the world's largest cave with an underground jungle and a river, has been opened to the public for the first time since its discovery in 2009.

The Son Doong Cave was discovered by British cavers, led by Howard Limbert, in 2009, eight years after a local man named Ho Khanh found it in 1991. Four years on, the cave was officially opened for tours in July and it will be hosting its first tourists, a batch of 220 adventure travellers, until the end of August 2014.

The Son Doong Cave which is located in the heart of the Phong Nha Ke Bang National Park in Quang Binh province near the Laos-Vietnam border is so large that a skyscraper could fit inside it, according to National Geographic.