[en] The ³⁷Ar activity in ground-level air samples from various countries (Switzerland, Denmark, USA, Canada, India, Australia, and Argentina) was monitored since 1969. Low-activity values of about 0.0025 dpm/l of argon were in agreement with the expected production rate by cosmic rays after mixing in altitude and latitude. However, high-activity values up to 1.8 dpm/l of Ar were observed several times. These were attributed to artificial sources, most probably to underground nuclear explosions. Estimates indicate that 0.7 to 2 MCi of ³⁷Ar must have been released into the atmosphere. Wind transportation and a two-dimensional multi-box eddy diffusion model were used to interpret the data in terms of atmospheric mixing. The best fit to the data was obtained with values between 1 and 3 x 10¹⁰ cm² s^-1 for an average latitudinal eddy diffusion coefficient between 20⁰ and 70⁰N. Minor variations in the ³⁷Ar activity may be due to ³⁷Ar released from reactors or fuel reprocessing plants