For most hikes, the destination -- whether it be the top of a dramatic cliff or the foot of a tall waterfall -- is the reward.
But then there are lava tube hikes, where passing through and exploring the mysterious and alluring caves is the reward.
Inside the Thurston Lava Tube on Hawaii's Big Island.
Lava tubes are cavernous tunnels that formed long ago when the outermost layer of a lava stream crusted, enclosing the fiery flow beneath. When the lava flow eventually drained by oozing through natural openings or rupturing through the surface, it left behind dark passageways with twists, turns and eerie formations.
From the lava fields of Hawaii to the lava river beds of Arizona, America has a variety of these fascinating grottos, each one unique in its own way.
Some lava tubes, like the Nāhuku tube on Hawaii's Big Island, are short, easy to access and surrounded by lush forests. Others, like the tubes beneath Lava Beds National Monument in California, are caves deep in the Earth, just waiting to be explored.
Just be careful, some lava tube hikes require a flashlight, hard hat, climbing gear or knee pads -- always come prepared and always exercise caution.
Below are seven lava tubes you can hike through, each one as eerie as it is beautiful.
1. Ka'eleku Caverns, Maui
These caves are a worthy detour off the island's scenic and remote Road to Hana.
2. Lava Beds National Monument, California
This remote park near the Oregon border holds the largest concentration of lava tubes in the United States.
3. Lava River Cave in Newberry National Volcanic Monument, Oregon
It takes 90 minutes to explore this mile-long cave south of Bend. Beware of bats!
4. Nāhuku (aka Thurston Lava Tube), Hawaii Island
A short and sweet walk in a lush forest takes you to one of the country's younger lava tubes.
5. Ape Cave, Washington
At 13,042 feet long, Ape Cave is the third longest lava tube in North America. It's just south of Mount St. Helens.
6. Coconino National Forest's Lava River Cave, Arizona
This 700,000-year-old lava tube near Flagstaff still has ripples frozen into the floor from the lava that flowed there long ago.
7. Kazumura Cave, Hawaii's Big Island
One of the longest and most intricate lava tubes in the world, Kazumura Cave has 40 miles of pure exploration bliss.
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