Mexico’s Giant Crystal Cave Is Beautiful But Deadly ... The hollow space of the cave was filled with this mineral-rich hot water and remained filled for about 500,000 years.

Located along a river, this forested park offers a historic cave and outdoor recreation in the heart of the Midwest.

The stalactites and stalagmites of Sequoia National Park’s Crystal Cave, a sprawling subterranean wonder that’s been closed for four years, will be accessible again this summer. But to get in ...