Why are deep underground or underwater locations ideal for studying the rarest, most energetic primary cosmic rays?

Answer

They filter out almost all secondary cosmic rays and background radiation

Placing detectors deep underground or underwater effectively filters out almost all secondary cosmic rays and terrestrial background radiation, allowing scientists to isolate and study the extremely rare, highly energetic primary particles.

Why are deep underground or underwater locations ideal for studying the rarest, most energetic primary cosmic rays?
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