X-rays began on Nov. 8, 1895 through Wilhelm Röntgen's work with tubes and radiation.
HowStuffWorks notes, "As with many of mankind's monumental discoveries, X-ray technology was invented completely by accident."
Also known as X-radiation, X-rays is a form of electromagnetic radiation.
Google's Marissa Mayer noted that the small 'g' in today's special doodle is a homage to PigeonRank, a past April Fool's joke at the company.
See the X-rays Google doodle below: