The Great Red Spot is an anticyclonic (high-pressure) storm on the planet Jupiter, 22° south of the equator; which has lasted at least 300 years. The storm is large enough to be visible through Earth-based telescopes. It was first observed either by Cassini or Hooke around 1665. The spot is red because it contains reddish methane-rich gases which rise from the lower atmosphere.
This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 5.7 million miles (9.2 million kilometers) from Jupiter. Cloud details as small as 100 miles (160 kilometers) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.
The oval object rotates counterclockwise, with a period of ~6 days. The Great Red Spot's dimensions are ~24-40,000 km x 12-14,000 km. It is large enough to contain two or three planets of Earth size. The cloudtops of this storm Great Red Spot are ~8 km above the surrounding cloudtops.
Storms such as this are not uncommon within the atmospheress of gas giants. For example, Neptune has the Great Dark Spot. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last hours or centuries.
It is not known exactly what causes the Great Red Spot's reddish color. One theory is that phosphorus compounds raised from deeper in the atmosphere by the Spot's updrafts are responsible, but this remains speculative. The Great Red Spot is remarkably stable, having first been spotted over 300 years ago. Several factors may be responsible for its longevity, such as the fact that it never encounters solid surfaces over which to dissipate its energy and that its motion is driven by Jupiter's internal heat. Simulations suggest that the Spot tends to absorb smaller atmospheric disturbances.
NOTE: First paragraph claims the red color is caused by methane; fourth paragraph says it's not known exactly why it's red. Which is correct?
At the time of writing, the Great Red Spot is approximately half as large as it was 100 years ago. It is not known how long the Great Red Spot will last, or whether this is a result of normal fluctuations.