Despite decades of reports of weak magnetic field effects in biology across the tree of life and on a broad range of cell types, the evidence to date remains met with skepticism. To remedy this, we present open-data, large-scale, and varied morphological evidence that Xenopus laevis embryo development is accelerated in a well-engineered, environmentally-calibrated hypomagnetic field of less than 1 nT. These data imply that basal tadpole physiology can sense and react to the absence of Earth’s minute magnetic field of approximately 50 μT. The effect is significant, as demonstrated by a variety of statistical measures. As no definitive biophysical mechanism has been identified to account for its occurrence, this study raises the question of which mechanism provides the most plausible explanation. How that question is answered may have implications in a variety of fields, including human health, behavioral ecology, and space exploration.