If you've ever driven to the Redmond airport, you've likely passed right by these caves without noticing them. Three caves provide access to one lava tube, formed when molten lava flowed through hardened lava rock. When the molten lava drained out of the tube, the open space remained. Caves formed when the top of the lava tube collapsed, leaving an entry point into the open space below the ground.
These caves are relatively short and easy to explore. The trailhead sign recommends bringing three sources of light into the cave; in case of failure, it's important to have backup.
From the parking area, Cave #4 is just to the right. This cave is relatively easy to enter. The ceiling becomes lower and lower as you walk deeper into the cave, and eventually it becomes impassable. Further down the trail, you'll see Cave #3. Walking along the main pathway, you can exit through another cave just north of where you came in! Step outside and turn right, where you can re-enter the earth through the final cave entrance.
Inside the caves you'll notice the traffic noise from the airport road completely disappears. The temperature is always moderate, and if you get far enough away from the entrance, it will be completely dark. It is a miniature escape just outside of the city.
The downside to easy access is that the cave walls are marred with decades of graffiti. Try and leave Redmond Caves cleaner than when you found it by packing out trash and reducing your impact.