The term ‘pingu’ is a reference to the nickname the Prince gained once he completed his training to become a badged Frogman.
The gruelling course is run across nine stages in nine months, and consists of a scuba diving course, a survival course and a combat swimmer course.
Out of the 500-600 applicants that start the course, a mere dozen are left after the nine months.
The 50-year-old royals grandfather, King Frederik IX who ruled Denmark from 1947 – 1972, had tattoos of dragons and birds on his torso.
And while Frederik has been quite successful in hiding his ink, they have been spotted by eagle-eyed royal fans.