The May 1921 geomagnetic storm was a significant event caused by the impact of an extraordinarily powerful coronal mass ejection on Earth's magnetosphere. It took place on 13 through 15 May, and was part of solar cycle 15. This event occurred before extensive interconnectivity of electrical systems and the general electrical dependency across infrastructures in the developed world, so the effect was restricted to certain sectors, even though resulting ground currents were up to an order of magnitude greater than those of the March 1989 geomagnetic storm that blacked out large parts of northeastern North America. At the time, scientists gave the size of the sunspot that began on May 10th and caused the storm as 94,000 by 21,000 miles (131,000 km by 33,800 km) in size.
Northern lights appeared in much of the eastern United States, creating brightly lit night skies. Telegraph service in the United States was slowed and then virtually eliminated around midnight of the 14th due to blown fuses, and damaged equipment. On the other hand, radio waves were strengthened during the storm due to ionosphere activation, allowing for some strong intercontinental reception. Electric lights did not seem to have been noticeably affected. Undersea cables also suffered from the storm. Damage to telegraph systems were also reported in Europe and the southern hemisphere.