The basics: Two of the planet's most active volcanoes bubble and emit plumes of volcanic gas inside this park on the island of Hawaii, reminders that forces deep inside the Earth are still at work. At the Jagger Museum, visitors can see gases rise from the Halema'uma'u Crater; after sunset, the vapors glow red, reflecting the lava lake that churns within the crater.
Visitors: 1.83 million people visited the park in 2015.
Square miles: 520.
More information: nps.gov/havo; 1-808-985-6000.