Why Do Japanese People Avoid Swimming After Obon?
In Japan, mid-August is set to the rhythm of cicadas chirping and the celebration of Obon (お盆), a sacred period stemming from Buddhist tradition where the spirits of ancestors return to earth to visit the living. While this festival is synonymous with summer dances (Bon Odori) and family reunions, it also marks a sharp turning point in the Japanese summer: the almost immediate end to swimming in the sea.
Is it a simple ancestral superstition or a reality dictated by nature? Let’s decipher a Japanese summer tradition.
The Legend: Spirits of the Depths (Muenbotoke and Yōkai)
In Japanese popular folklore, venturing into the water during or right after Obon (around August 15) is perceived as major recklessness. According to belief, the boundaries between the world of the living and that of the dead thin out.
Tradition tells that the muenbotoke (無縁仏)—wandering souls who no longer have family to honor their memory or maintain their grave—hide beneath the waves. Jealous of the living, these spirits, sometimes associated with yōkai (supernatural aquatic creatures), attempt to grab swimmers’ feet to drag them into the abyss and take their place.
This terrifying legend historically served as a remarkably effective warning to keep children away from the water at a pivotal time of the year.
Meteorological Reality: The Arrival of Typhoon Season
Beyond the myth, the scientific explanation gives full merit to this customary ban:
- A fatal peak in visitors: The national Obon holidays push millions of vacationers to the coasts. Mechanically, this summer overpopulation at the water’s edge leads to a statistical surge in accidents and drownings, which may have reinforced popular belief in malevolent spirits.
- The danger of rip currents: Mid-August signals the intensification of the typhoon season. Even if a storm hits far offshore, it generates what is called doyō-nami (late-summer high waves) and very powerful rip currents known as riganryū (離岸流) along the Japanese coast. These currents, sometimes invisible on the surface, can carry away an experienced swimmer out to sea within seconds.
Kurage: The True Guardians of Late Summer
If spirits and high swells aren’t enough to dissuade you, marine life will take care of it. In Japan, jellyfish (kurage / クラゲ) mark the true end of the beach season.
Right after Obon, warming waters and shifts in marine currents trigger a massive arrival of jellyfish near the beaches. Among them, the box jellyfish (Andon kurage / アンドンクラゲ) is particularly feared. Translucent and very difficult to spot, it inflicts extremely painful stings comparable to electric shocks. Their proliferation is the main practical reason Japanese beachgoers put away their swimsuits starting August 16.
Did You Know? The Mystery of Freshwater Jellyfish (Mamizu-kurage)
While jellyfish are always associated with the sea, Japan is home to a fascinating species that lives inland: the Mamizu-kurage (マミズクラゲ / Craspedacusta sowerbii), literally “freshwater jellyfish.”
These tiny creatures, about the size of a 100-yen coin (roughly 1 to 2 centimeters), exhibit ghostlike and unpredictable behavior. For most of their lives, they remain in the form of microscopic polyps attached to the bottom of the water. It is only at the end of summer or early autumn that they briefly metamorphose to swim freely.
In the Kansai region, their sporadic appearances can be observed in certain calm bodies of water, such as the Saigahara pond located in the mountains of Minoh. They have also been spotted in ponds around Kobe, where the Suma Aquarium has even held temporary exhibitions to present these rare aquatic specimens to the general public.
Whether to avoid disturbing ancestors, to guard against treacherous currents, or to flee the painful stings of kurage, the Japanese custom of no longer swimming after Obon remains, even today, advice to be followed to the letter!
