Paris -- The rooftop of a Paris skyscraper was transformed into a cosmic-ray laboratory in an unusual week-long experiment that started Saturday.

Every time a detector on top of the 210-metre (689-feet) Montparnasse Tower picks up a sub-atomic particle called a muon, a pulse of laser light will flash across the sky of the city's Latin Quarter from the Paris Observatory.

Muons are debris from protons that are blasted out from the Sun or beyond our Solar System and constantly bombard the Earth.

The protons smash apart when they collide with molecules in the upper atmosphere. Their short-lived remains shoot down to the planet's surface at nearly the speed of light.

The so-called "cosmic opera" seeks to inform the public about cosmic particles and pay tribute to an experiment into the phenomenon, conducted at the top of the Eiffel Tower in 1910 by a German physicist, Theodor Wulf.