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Sources and sinks of methane (CH<sub>4</sub> ) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH<sub>4</sub> concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH<sub>4</sub> . Recently, it was hypothesized that this CH<sub>4</sub> depletion may be caused by radiolysis, an abiotic process whereby CH<sub>4</sub> is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH<sub>4</sub> concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH<sub>4</sub> in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH<sub>4</sub> at a rate of 1.3-2.7 mg CH<sub>4</sub> · m<sup>-2</sup> · d<sup>-1</sup> . These CH<sub>4</sub> oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH<sub>4</sub> in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH<sub>4</sub> not only in caves, but also in smaller-size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH<sub>4</sub> oxidation scales up to affect local, regional, and global CH<sub>4</sub> cycling.