It's the height of summer in the hot part of Oregon. The steel stairs lead us down the hole in the earth over the uneven jumble of shifting volcanic boulders. The bright heat shifts to a cool blackness. The temp changes quickly as we enter the gaping mouth of the lava tube. We follow the stairs deeper underground and soon have to switch on our headlamps in the oily black.
There is no light source in the cave save for the quickly disappearing entrance. The dirt beneath our feet feels different and there's a cool earthy smell. We shuffle our feet along at first, unacquainted with the darkness and volcanic terrain. In the distance there are voices, but we can only see as far as our lamps and feel alone.
Our other senses wake up, and we can tell that we are in big room. Our voices bounce back in big echoes.
I take photos with the flash and stand I fumble to carry. Each burst of light reveals distinguishing features of the volcanic rock hidden to our naked eyes. The 50 foot ceiling creates a grand throne room. 
The lava tube leads us deeper at an easy descent. We realize our voices bounce back to us more quickly, the tube narrowing around us. At some places we duck so as not to bump our heads.
We get to the end and returned to our favorite joke "Where's the snack bar?" It's especially ironic in the end of a black tunnel. The tube had slowly closed in around us, standing only at 6 feet now. We turn around and begin the walk back the way we came - so deep into the earth that no light at the end of tunnel leads our way out.
To turn off the headlamp is to be bathed in indistinguishable darkness. The claustrophobic kind.
We walk the mile back in the cool cellar temp of 42 Fahrenheit (5 Celsius). The stairs eventually reappear, silhouetted in the bright hot summer, bringing us back to the real world.
The Lava River Cave is located near Bend, Oregon. It a few feet shy of a mile in length (1588 m). At it's deepest, it's 50 feet below the ground. It's said to be discovered by a pioneer hunter chasing a wounded deer, however it's more likely that native American's made good use of it's cooling properties long prior, by evidence of the obsidian flakes.
The cave (or tube) was created by a river of flowing lava. As the lava cooled on top it created a ceiling to a river of moving hot lava below. Rather than the rest of the lava cooling to form a solid mass, it drained out, leaving the cave behind. Eventually, the portion near ground level collapsed, providing an opening for us humans to wander below the earth's surface. This particular tube is the longest continuous tube in Oregon. The longest tube, as far as we know, is in Hawaii and is 40 miles long (64 kilometers)! That sounds scary!
If you're ever in the Bend area, this a great way to spend 2-3 hours. It's high on my list because I like to wander unattended, and you can do this at this site! Bring your own headlamps, or you can rent them at the site. Be aware that cold can drain batteries and if you're especially scared of the dark, you might want to bring backups. Temps underground remain mostly constant, so you can expect 42 Fahrenheit (5 Celsius) if it's summer or winter! It may be a more a welcome respite in the summer. Would you explore this lava tube?