  At the beginning of the twentieth century, two scientists, the Austrian
Victor Hess and the Italian Domenico Pacini, developed two brilliant lines of
research independently, leading to the determination of the origin of
atmospheric radiation. Before their work, the origin of the radiation today
called "cosmic rays" was strongly debated, as many scientists thought that
these particles came from the crust of the Earth.
  The approach by Hess is well known: Hess measured the rate of discharge of an
electroscope that flew aboard an atmospheric balloon. Because the discharge
rate increased as the balloon flew at higher altitude, he concluded in 1912
that the origin could not be terrestrial. For this discovery, Hess was awarded
the Nobel Prize in 1936, and his experiment became legendary.
  Shortly before, in 1911, Pacini, a professor at the University of Bari, made
a series of measurements to determine the variation in the speed of discharge
of an electroscope (and thus the intensity of the radiation) while the
electroscope was immersed in a box in a sea near the Naval Academy in the Bay
of Livorno (the Italian Navy supported the research). The measures are
documented in his work "Penetrating radiation at the surface of and in water".
Pacini discovered that the discharge of the oscilloscope was significantly
slower than at the surface.
  Documents testify that Pacini and Hess knew of each other's work. Pacini died
in 1934, two years before the Nobel Prize was awarded for the discovery of
cosmic rays. While Hess is remembered as the discoverer of cosmic rays, the
simultaneous discovery by Pacini is forgotten by most.
