WASHINGTON (AP) -- The conclusive discovery by a pair of wheeled robots that Mars once had vast pools of water and possibly could have harbored life was chosen by the editors of the journal Science as the most important scientific achievement of 2004.

NASA's two Mars rovers, Opportunity and Spirit, landed on the Red Planet early in 2004 and have since found clear and conclusive evidence that Mars was drenched with water at some time in its history.

The editors of Science, one of the world's leading publishers of peer-reviewed, original research, judged the robotic accomplishment as the top scientific "Breakthrough of the Year."

"Inanimate, wheeled, one-armed boxes roaming another planet have done something no human has ever managed," Science reported in this week's edition. "They have discovered another place in the universe where life could once have existed."

Nine other scientific achievements, including discovery of another species of human, were selected as runners-up, but Science editor-in-chief Donald Kennedy said "there wasn't much doubt about this year's winner."

Opportunity and Spirit found unmistakable proof of Martian water: rippled sediments that were once at the bottom of a shallow sea, and rock that once was so water-soaked that "it had rotted," the journal said.

"Their finds mark a milestone in humankind's search for life elsewhere in the universe," Science said.

Kennedy said one of the most important messages from the remote exploration is "the extraordinary efficiency of these robot missions."

He said it is clear that the National Aeronautics and Space Administration must not abandon its robotic exploration while gearing up for President Bush's program to send humans to the moon and later to Mars.

"To do one at the expense of the other would be a mistake," Kennedy said. "It remains to be demonstrated what a human exploration could do that we can't do now or couldn't do in the next 10 years with robotic technology."

The first runner-up for breakthrough of the year was the discovery on the Indonesian island of Flores of fossils from a species of tiny humans who stood about 3 feet tall and had a brain less than a third the size of modern humans. Yet, the diminutive hominid lived about 18,000 years ago. This suggests that Homo floresiensis shared the Earth with Homo sapiens, or modern people. Science said some described the find as "the biggest discovery in half a century of anthropological research."

Third on Science's list of 2004 breakthroughs was the cloning of human embryos by South Korean researcher Woo San Hwang and his colleagues. The work was not an attempt to genetically duplicate a human. Instead, the researchers hoped to make embryonic stem cells for research purposes. Although many other mammals have been cloned, the work was the first to demonstrate that cloning techniques would work with human cells.

Following are the rest of Science's 2004 selections.

4.       U.S. and Austrian scientists created a new form of condensate, an ultracold gas that slips into a quantum state where a group of atoms act as a single superatom. The achievement was notable because it used fermions, a class of atoms with a nuclear structure that makes it difficult to create a condensate.

5.       Scientists discovered that "junk DNA," the base pairs between known genes in the human genetic structure, play an important role. Several research teams have found that DNA between genes helps determine how vigorously and often the genes are activated and shapes the coding for protein production.

6.       Astronomers discovered a pair of neutron stars locked in orbit of each other and spewing out beams of radiation. Both objects are pulsars, rapidly flickering on and off with pulses of energy. One object is pulsing at the rate of 44 times a second. By studying the radiation, astronomers hope for the first time to learn about the density of matter within a neutron star.

7.       Naturalists tracking the fate of wild species worldwide reported bad news. A survey of amphibians found that of 5,700 known species, about 30 percent were at risk of extinction. A survey in the United Kingdom found that butterflies, songbirds and native plant species are all losing ground in the battle for species survival.

8.       It is one of the most common and universally known substances, but researchers are still learning more about water. Several teams of researchers made new discoveries about how water molecules bind together and how electrons and protons dissolve in water. Some of the findings are questioned and Science noted: "Water still gives researchers much to scratch their heads about."

9.       A new form of research and aid is creating "a revolution in public health," said Science. The partnership of public and private organizations worldwide is changing the way drugs are developed, tested and distributed to the poorest nations on Earth, the journal said. Researchers tallied at least 92 public-private partnerships worldwide attacking such diseases as malaria, tuberculosis and HIV.

10.    Researchers have developed techniques to identify genes in ocean water or in specimens recovered from deep underground. Thousands of new genes have been found. By sequencing these genes, researchers hope to identify news species and, perhaps, learn how organisms survive in harsh and forbidding locations on Earth.