Ice in a lava tube at Lava Beds National Monument, California

© Floris van Breugel/Minden Pictures

Ice in a lava tube at Lava Beds National Monument, California

It gets cold enough at this volcanic landscape in northern California to freeze the water that collects and flows through some of the caves and lava tubes, creating other-worldly visuals. The caves and tubes suitable for human exploration are rated from least to most challenging, with the easiest requiring only good boots, head protection, and a reliable flashlight. The more challenging caves are often accessible only by stairs or ladders. Think of all the cool photos you’ll get.