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<p>Methane (CH<span class="inline-formula">₄</span>) seepage (i.e., steady or episodic flow of gaseous hydrocarbons from subsurface reservoirs) has been identified as a significant source of atmospheric CH<span class="inline-formula">₄</span>. However, radiocarbon data from polar ice cores have recently brought into question the magnitude of fossil CH<span class="inline-formula">₄</span> seepage naturally occurring. In northern high latitudes, seepage of subsurface CH<span class="inline-formula">₄</span> is impeded by permafrost and glaciers, which are under an increasing risk of thawing and melting in a globally warming world, implying the potential release of large stores of CH<span class="inline-formula">₄</span> in the future. Resolution of these important questions requires a better constraint and monitoring of actual emissions from seepage areas. The measurement of these seeps is challenging, particularly in aquatic environments, because they involve large and irregular gas flow rates, unevenly distributed both spatially and temporally. Large macroseeps are particularly difficult to measure due to a lack of lightweight, inexpensive methods that can be deployed in remote Arctic environments. Here, we report the use of a mobile chamber for measuring emissions at the surface of ice-free lakes subject to intense CH<span class="inline-formula">₄</span> macroseepage. Tested in a remote Alaskan lake, the method was validated for the measurement of fossil CH<span class="inline-formula">₄</span> emissions of up to 1.08 <span class="inline-formula">×</span> 10<span class="inline-formula">⁴</span> g CH<span class="inline-formula">₄</span> m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span> (13.0 L m<span class="inline-formula">⁻²</span> min<span class="inline-formula">⁻¹</span> of 83.4 % CH<span class="inline-formula">₄</span> bubbles), which is within the range of global fossil methane seepage and several orders of magnitude above standard ecological emissions from lakes. In addition, this method allows for low diffusive flux measurements. Thus, the mobile chamber approach presented here covers the entire magnitude range of CH<span class="inline-formula">₄</span> emissions currently identified, from those standardly observed in lakes to intense macroseeps, with a single apparatus of moderate cost.</p>