Yellowstone National Park, WyomingLorcel/Shutterstock
Arguably the most impressive of all U.S. National Parks, Yellowstone sits squarely over an active supervolcano. Created by a massive volcanic eruption some 640,000 years ago, the Yellowstone caldera now covers an area measuring a whopping 30×45 miles. The heat beneath this area is responsible for the park’s many interesting hydrothermal features—geysers, including Old Faithful, fumaroles, hot springs, and bubbling mud pots—that are all huge tourist draws. Below ground, earthquakes are happening all the time (1,000 to 3,000 per year!) which the University of Utah Seismograph Station tracks closely.
Arenal, Costa RicaNancyS/Shutterstock
The centerpiece of 30,000-acre Arenal Volcano National Park, Costa Rica’s most active volcano is also one of the country’s top tourist attractions. Lava has flowed since Arenal last erupted in 1968 and wiped out two nearby towns. By day, you can see smoke and ash billowing from Arenal’s peak. Come nightfall, you can watch the fiery-red lava ooze down its western slope. You can take a guided hike at the base of the volcano; park rangers monitor Arenal’s activity and close the trails when there’s a safety concern.