INTRODUCTION We aimed to investigate how, changes in atmospheric pressure influence the human body. METHODS The study was an observational study, reported according to the STROBE-guideline (STrengthening the Reporting of OBservational studies in Epidemiology). Participants had their abdominal circumference measured on ground level, at the bottom of a pool, and during flight. This was used to investigate the relationship between atmospheric pressure and abdominal circumference. RESULTS We included 17 participants for the flight study and 12 participants for the pool study. Flying increased abdominal circumference from median 82 cm (range: 72-117 cm) at ground level to 86 cm (74-122 cm) in flight, p = 0.001. Submersion in water caused a decrease in abdominal circumference from median 82 cm (70-116 cm) at ground level to 79 cm (67-114 cm) under water, p = 0.003. Furthermore, flying resulted in a significant increase in self-reported flatulence and bloating from median 27 (0-69) to 50 (0-93), p = 0.02 (scale 0-100). Flying also caused an increase in abdominal pain from median 0 (0-68) to 3 (0-70), p = 0.02 (scale: 0-100). CONCLUSION Our findings confirm that changes to environment and thereby atmospheric pressure, influence abdominal circumference. This is likely due to expansion and compression of bowel gasses related to the ambient atmospheric pressure. Interestingly, our findings may help explain the physical appearance of mermaids. FUNDING none. TRIAL REGISTRATION none.