This striking satellite photo shows an exceptionally dense and toxic algal bloom swirling in the waters of Nevada’s Pyramid Lake. Some experts believe that the algae has existed in the lake for at least 9,000 years.

Pyramid Lakeis a roughly 175-square-mile (453 square kilometers) lake, located northeast of Reno. It is around 27 miles (43 kilometers) across at its widest point and is named after a large pyramid-like rock formation located on a small island in the lake’s southern half.

The lake is fed by a single river, but it is endorheic, meaning it has no outflow. As a result, it has a much higher salinity than most lakes, because excess water evaporates away, leaving its dissolved salts behind. The lake is also slightly alkaline and has a pH of around 9, which is roughly equivalent to baking soda.

Pyramid Lake experiences annual algal blooms between September and October, due to warmer temperatures and increased nutrient availability. The predominant algal species isNodularia spumigenawhich is a blue-green, nitrogen-fixing cyanobacteria that thrives in high-salinity waters, according toNASA’s Earth Observatory.

But 2024’s bloom, captured in this satellite image, was one of the most extreme in recent memory. It peaked on Oct.

15, around a week after this photo was taken. It is currently unclear why this bloom was so intense.

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N. spumigenahas the potential to produce toxins, such as nodularin, which can affect the liver and cause other adverse health effects in humans and other animals. During the 2024 bloom, local authoritieswarned peopleto not let their pets anywhere near the water.

Pyramid Lake was once part of a much larger, prehistoric body of water, known as Lake Lahontan, which covered an area of around 8,000 square miles (21,000 square km) — around 45 times larger than Pyramid Lake.

This ancient lake was hit by climate change at the end of the last ice age, orPleistocene epochwhich ended around 12,000 years ago. Around 3,000 years later, Lahontan had essentially disappeared, leaving Pyramid Lake as its largest remaining remnant.

Today, Pyramid Lake is known for a series of striking pillar-like calcium carbonate features, known as tufas, which grew from Lake Lahontan’s receding waters between 26,000 and 13,000 years ago, according to theU.S. Geological Survey. (This includes the pyramid-like structure the lake is named after.)

A1990 studyanalyzed trends across 15 consecutiveN. spumigenablooms in Pyramid Lake between 1972 and 1986. The researchers theorized that the cyanobacteria once bloomed in Lake Lahontan and may have persisted in its remaining waters ever since.

Pyramid Lake is also home to the cui-ui (Chasmistes cujus) — an endangered species of suckerfish that isn’t found anywhere else on Earth. These fish feed mainly on algae, such asN. spumigenaand also likely used to dwell within Lake Lahontan.