Methane hydrates, ice-like compounds that consist of water and methane, represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification; however, it remains unclear how much hydrate exists. Here, using a newly developed three-dimensional (3-D) thermal technique, we reveal a novel method for detecting and quantifying methane hydrate. The analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter-scale horizontal resolution. Our study, located at Hydrate Ridge, offshore Oregon (United States), suggests that heat flow and hydrate concentrations are coupled and that 3-D thermal analysis can be used to constrain hydrate and fluid flow in 3-D seismic data. Hydrate estimates using this technique are consistent with 1-D drilling results, but reveal large, previously unrecognized swaths of hydrate-rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies. The 3-D analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three.