NASA's Landsat 8 satellite constantly photographs the Earth. On August 11, 2015, it captured this section of the Baltic Sea.
Researchers saw what they think is a beautiful bloom of phytoplankton, made mostly of microscopic plants.
It stretches for hundreds of miles across the sea.
"We steamed through ‘carpets’ of it the entire day," one scientist on a research ship told NASA.
The researchers suspect the bloom is full of cyanobacteria, also called blue-green algae, which absorb sunlight like a plant.
Also like a plant, they generate oxygen. Most of Earth's oxygen probably comes from cyanobacteria.
But sometimes the beauty of blooms can be deceiving.
Too many algae can overload waters: As they die, other bacteria eat them and use up oxygen in the water. This can create oxygen-depleted dead zones where "organisms cannot survive."
Other species of algae produce toxins that can sneak into fishes and shellfish, threatening the food supply.
Blooms of harmful algae are suspected in mass die-offs of other animals, including a recent killing of dozens of whales near Alaska.
Meanwhile, cruise ships are reportedly still dumping sewage into the Baltic Sea. Along with fertilizers in agricultural runoff, this fuels bloom growth.
The Helsinki Convention of 1974 banded countries together to curb Baltic Sea pollution, but it may not be enough.
Climate change is draining more freshwater into the Baltic, which further encourages blooms to grow. So giant blooms appear somewhat regularly during the summer.
Addressing the problem will likely take the whole planet working together to reduce greenhouse emissions, slow climate change, and curb its disruption.
