Astronomers say that intense magnetic fields growing above the sunspot are being twisted into an unstable configuration that may produce a powerful burst of energy or flare.
The largest spot seen on the Sun for nine years has moved into view.
The spot, which is moving towards the centre of the Sun's disc, covers an area a dozen times larger than the entire surface of the Earth.
It comes at a time when the Sun is going through the maximum phase of its 11-year cycle of activity.
Astronomers say that intense magnetic fields growing above the sunspot are being twisted into an unstable configuration that may produce a powerful burst of energy or flare.
Sunspots are believed to be caused when magnetic fields, shaped into huge coils, emerge on to the solar surface from below.
Surface gas is cooled slightly and appears darker than the surrounding hot gas. If a sunspot was on its own, it would be incredibly bright.
Although the latest sunspot - in solar active region 9169 - is exceptionally large, it is not a record-breaker, according to Dr David Hathaway, of Nasa's Marshall Space Flight Center in the US.
Power lines knocked out
The largest known sunspot was seen in 1947, and was three times larger than this latest feature.
A large sunspot in March 1989 triggered a disturbance in the Earth's magnetic field that knocked out power lines in Canada.
Astronomers measure the sizes of sunspots as fractions of the Sun's visible area. The current sunspot is 2,140 millionths of this area.
By comparison, the entire surface area of the Earth is only 169 millionths of the Sun's visible hemisphere.
One of the best close-up images of the sunspot was obtained by amateur astronomer Franky Dubois from his private observatory in Belgium.
Mr Dubois told BBC News Online that it was one of the most spectacular sunspots he had ever seen.
Solar telescopes around the world have already focused on the sunspot to analyse its detailed structure and how it will change over time.
Astronomers are keen to point out that to observe sunspots care is needed.
They say no-one should ever look directly at the Sun under any circumstances, with or without optical aids, as this can cause blindness.