The Bizarre Ancient Rituals Involving The Nile River
Writing in the 5th century A.D., Greek historian Herodotus famously observed that the nation of Egypt was a "gift of the river," per Project Gutenberg, and it's not hard to see why. The world's first nation-state (via BBC), Egypt, was built in the middle of a desert in a literal oasis. The Nile River supplied the entire nation with essentially everything it needed to sustain life: both water and fertilizer, in the form of its silt (via Carnegie Museum of Natural History). It's absolutely no exaggeration to say that, without the Nile, there would have been no nation of Egypt, which is why the river figured so prominently into ancient Egyptian folklore, mythology, and religion.
It was also why there were so many bizarre rituals involving the Nile — for example, Egyptians would occasionally build mud sculptures of their god Osiris studded with barley seeds on the banks (via BBC). Even more strange, once a year, all the men would gather at the river, strip naked, and masturbate into it.
Yep, the bizarre ancient ritual was apparently a real thing
In the modern Western world, group activities like these are mainly just a lazy punchline for movies (and/or a source for rap-metal band names), but different cultures have had wildly different understandings of the act. At one extreme, there are certain remote tribes in sub-Saharan Africa that had — until recently — never encountered this form of self-pleasure, and find the idea of it completely bizarre (see a study here); at the other, you have civilizations like Ancient Egypt, which thought of the act as literally sacred (via MEL).
This was actually embedded into the mythology of Ancient Egypt: The very first god, Atum, was said to have initiated creation by using his "seed" to produce the gods Shu and Tefnut (via Victor H. Matthews and Don C. Benjamin). Given this understanding, it should be no surprise that the act was understood as a life-giving one that symbolized the fertility the Nile would bring when it flooded every summer.
As such, an ancient ritual was performed once a year by the Egyptian Pharaoh, who would walk down to the shore, strip naked, and ceremonially pleasure himself over the water. Once he finished, all the other men would follow suit (via Face2Face Africa). Then, presumably, they would all spend the remainder of the year struggling to look each other in the eye.