Proximity Fuze
It was considered so secret that in the beginning it was only allowed to be used over the open sea just in case any failed they would be lost and not fall into enemy hands. The proximity fuze has since the end of the war been considered the 3rd most influential technological development of WW2 with its effect in battle both in the Pacific and Europe as greater than atomic bomb which was only used twice compared to the over 20 million proximity fuzes which were produced. This is the story of how they worked and how they change the course of the war.
Credit Curious Droid