The Underground River stretches for about 8.2 kilometers (5 miles) and is one of the longest navigable underground rivers in the world. However, only a portion of it, about 4.3 kilometers (2.7 miles), is open to tourists. Visitors typically embark on guided boat tours that take them deep into the cave, where they can marvel at the spectacular rock formations, stalactites, and stalagmites illuminated by spotlights.

The tour typically starts in the evening, with visitors boarding small boats that glide silently along the calm waters of the river. As darkness falls, the fireflies gradually emerge, illuminating the mangrove forest with their bioluminescent glow. The sight of these tiny insects twinkling like stars against the backdrop of the night sky is truly enchanting and is often described as a magical experience.