On our first day of field work, we went out to Arco Tunnel, one of the longest lava tubes at Craters. Arco Tunnel is of interest to us because it’s no longer accessible to the public, so there’s less risk of contamination, and there’s no light inside, so we can focus in on non-phototrophic microbes. We were fortunate to have a great guide from the National Parks Service, two interns from the Bureau of Land Management, and Kate with us—Arco Tunnel is difficult to navigate, and we definitely needed help to avoid getting lost! After crawling and scrambling throughout the day, we found some great sample locations and collected from 11 different sites throughout the cave. Temperature and humidity also varied widely throughout the cave, from around 10 ºC (~50 ºF) to 18.8 ºC (~66 ºF) in a room closer to the surface. Hopefully this contextual data will help us in our analysis, both to understand gradients within Arco Tunnel and to compare to the sites from our other field days. It’s amazing to see the diversity of colors and textures that exist in the caves, from white crystalline mineral deposits on the ground to yellow biofilms clinging to stalactites on the ceiling. The range of microenvironments we observed in the cave likely helps facilitate some of this diversity, but it’s incredible that despite no access to light and very few sign of other life, so many populations of microbes have found their own way to survive in the cave. We’re excited to see what results we get from sequencing these samples so that we can understand who is surviving there, and how!