Popular underground river, a new wonder in Philippines
The Puerto Princesa Underground River in the Philippines was named one of the new seven wonders of nature last year - and its keepers intend to keep it wondrous.
So visitor numbers are carefully capped at 900 a day and permits must be arranged in advance.
Such controls will protect the fragile fantasy landscape of river caverns, which are filled with limestone shapes that evoke the Titanic, waves and medieval cities.
These shapes are still slowly changing, century by century, while explorers are still mapping the complex river system.