Geophysicists say they have discovered a force that strengthens the pull of gravity, suggesting that Newton's 301-year-old law of gravity might be wrong.

"We're watching history happen," said C.F. "Chick" Keller, head of the Los Alamos branch of the University of California's Institute of Geophysics and Planetary Physics.Scientists from Los Alamos National Laboratories, which conducted experiments last year down a 11/4-mile bore hole on the Greenland ice sheet, said Monday their discovery may be a theoretical fifth force of nature, or a component of gravity.

The four forces are gravity, electromagnetism, the strong and weak nuclear forces.

"Either way, we're saying something pretty big, something you don't say every day in physics," Mark Ander, the Los Alamos geophysicist who reported the findings, told The Washington Post. "We're saying we appear to have the cleanest evidence to date of something that cannot be explained by Newtonian gravity."

A change in the understanding of gravity may help physicists formulate a long-sought unified field theory that could explain all the forces of the universe in a single equation. It also might lead to recalculation of the masses of the stars and planets, and new estimates of the universe's age.

"Gravity could be more complicated than Newton or even Einstein thought," said Richard Hughes, a physicist at Los Alamos.

Electromagnetism is the force that holds atoms together; the strong nuclear force holds atomic nuclei together, and the weak force causes radioactive decay.

Ander, one of the two chief investigators on the Greenland tests, said the new force has been measured over distances ranging from 1,600 to 5,500 feet, enhancing gravity by up to 3.9 percent.

Sir Isaac Newton suggested in 1687 that gravity causes all bodies to attract each other and that the attraction increases as the weight of the objects increase and the distance between them decreases. But he also concluded that gravitational acceleration is always the same.