This article was originally published in the July/August 2016 issue of Popular Science, under the title “I Created a Vortex Ring in the Pool.” More deals, reviews, and buying guides ...

On hot days like this one in 1955, Black swimmers thronged to Pool No. 2, a segregated pool at Druid Hill Park in Baltimore, Maryland. Built in 1921, it was the only pool that accommodated the ...