TOKYO - A pond at Inokashira Park, a popular destination in Tokyo's Kichijoji area, has turned a striking matcha-like green, a phenomenon that has become particularly noticeable in 2026 and is believed to be caused by an accumulation of phytoplankton.
The pond at Inokashira Imperial Gift Park, which spans Musashino and Mitaka cities, covers nearly 42,000 square meters.
Shinichi Nakano, a professor at Kyoto University's Center for Ecological Research, said the phenomenon significantly affects the appearance of the pond but does not pose a health risk simply from viewing it.
However, accumulated phytoplankton can produce a foul odor when it decomposes, making it preferable to address the problem before that occurs.
Nakano said the only effective measure would be to drain the pond completely and reduce nutrients in the mud that provide food for the phytoplankton.
Inokashira Park has previously carried out complete pond-draining operations, but such work is large-scale and costly.
Experts say draining the pond about once a year would be desirable, but the expense means local authorities must consider how best to manage the problem.
Similar dramatic changes in the color of water and landscapes caused by microorganisms and natural processes can be seen in other parts of Japan and around the world.
In Numazu, Shizuoka Prefecture, the sea has been filmed glowing bluish-white at night.
The light is produced by marine plankton known as Noctiluca, which emit light under certain conditions. While several theories exist about why the organisms glow, the light is believed to serve as a defense mechanism.
According to Awashima Marine Park, the phenomenon can be observed when various environmental conditions align. The increase in Noctiluca is believed to have been encouraged by heavy rain in April, which washed nutrient-rich soil into the sea and contributed to plankton growth.
Although the glowing water creates a beautiful nighttime spectacle, excessive plankton growth can become a problem for people working in the fishing industry.
Microorganisms are also responsible for the extraordinary colors of Grand Prismatic Spring in Yellowstone National Park in the United States.
The hot spring displays a spectrum extending from deep blue at its center to green, yellow and orange.
According to the U.S. National Park Service, vast numbers of microorganisms invisible to the naked eye create the appearance of broad bands of color. Different microorganisms live at different temperatures, and their pigments produce the changing colors around the spring.
Another striking natural landscape is Rainbow Mountain in the Andes of Peru, known for its vivid bands of red, green and yellow.
According to the website of a local tour operator, the mountain began gaining recognition as a tourist destination around 10 years ago.
Smithsonian Magazine has reported that people living nearby said the mountain had previously been covered by snow, but warming temperatures caused the snow to melt and reveal the colorful slopes underneath. The spectacle has since attracted increasing numbers of visitors.
A different type of natural color phenomenon was recorded in Shark Bay, Australia, in March 2026, when the sky and surrounding landscape appeared to turn an intense blood-red.
The extraordinary scene was not created using artificial intelligence or a camera filter.
The color came from iron oxide, or rust.
The geology of the Shark Bay region contains large amounts of iron and naturally has a reddish-brown appearance.
As a cyclone approached the area in March 2026, strong winds lifted the iron-rich soil into the atmosphere, creating clouds of red dust.
The airborne dust mixed with sunlight, producing the intensely red landscape.
The phenomenon continued for about an hour.
Source: TBS
