FOR years, humans have known more about the surface of Venus than about what lies beneath their own oceans.
This week, scientists lifted the veil off of 70 percent of Earth's surface with a new map of the ocean floor that is expected to have an impact on activities ranging from the study of plate tectonics and the history of Earth's crust. Coming at a time when new technology is allowing scientists to probe farther beneath the surface than ever before, the cartography will aid the search for oil, minerals, and new fishing grounds in what may be the richest - and most hostile - environment on Earth.
Ocean scientists are greeting the images compiled by David Sandwell of the Scripps Institution of Oceanography in La Jolla, Calif., and Walter Smith of the National Oceanic and Atmospheric Administration's Geosciences Laboratory, with an awe usually reserved for the discovery of a distant planet.
''The significance of these data is profound,'' says Charles De Met, a geophysics professor at the University of Wisconsin at Madison. ''This is the first time anybody has seen the world's ocean floor at this level of detail. We're finally seeing what 70 percent of the Earth's surface looks like.''
''The data we had before was a leap forward in its own right. But this in an order-of-magnitude better than that,'' adds Marcia McNutt, director of the MIT/Woods Hole Oceanographic Institution Joint Program in Oceanography and Ocean Engineering.
Already the map is yielding what Drs. Smith and Sandwell call the ''definitive confirmation of the theory of plate tectonics.'' It is also yielding surprises. The scientists point to a junction of three crustal plates in the Indian Ocean, noting that the new view shows the plates moving in somewhat different directions than previously thought.
The new images stem in part from broader efforts to declassify data from military satellites. In February, for example, the US government began releasing photographs from its Corona satellite-reconnaissance program, which ran from 1960 through 1972. Some 886,000 images are expected to be made available through the National Archives and the US Geological Survey. When added to the collection of remote-sensing photos taken by civilian Landsat satellites, the Corona images will extend by more than a decade the span of time scientists can track changes in surface features such as vegetation, glaciers, river flows, and volcanic activity.
In addition, the US and Russia are working out the details of an agreement to exchange 30 years' worth of cloud-cover images from spy satellites. The photos would be a boon to climatologists. If successful, the pact could pave the way of an exchange of spy-satellite data to help warn of volcanic activity and help researchers study earthquakes. Negotiators hope to have the agreement ready for signing in December.
The new sea-floor map's release also comes as oceanographers, marine biologists, and marine geophysicists are marshaling for a long-term effort to study Earth's last frontier.
Using new generations of deep-sea submersibles, researchers hope to explore geological features that dwarf their dry-land counterparts. Canyon-like trenches, where vast sections of Earth's crust plunge back into the mantle, reach depths that exceed the cruising altitude of airliners.
Elsewhere, new material wells up from the mantle, forming fresh crust along midocean ridges. Both extremes - perhaps the most hostile environments on Earth - have given rise to bizarre life forms, some of which are giving researchers insights into how organisms developed during Earth's early stages. In addition, volcanic and tectonic activity over hundreds of millions of years have laced the sea floor with deposits of commercially valuable minerals such as cobalt, zinc, lead, and copper.
But to make the most of dwindling research dollars, it pays to know where to look.
''I've got an 55-day expedition coming up to conduct studies of undersea volcanoes among the austral islands south of Tahiti and southwest of Easter Island. This is the middle of nowhere,'' Dr. McNutt says. ''With this map, I've got a much better idea of where the volcanoes are and where I should place my instruments.''
For researchers from smaller universities with tighter budgets, the data represent a watershed - allowing world-class research from the comfort of one's own office, scientists say.
''It costs $1 million a month to go to sea, and you have to coordinate with two or three institutions. It can take years,'' says Juan Lorenzo, assistant professor of marine geology at Louisiana State University. His own work focuses on looking for natural resources on continental margins off northeastern Australia.
At the same time, he says, he is trying to build research opportunities for graduate and undergraduate students studying with him. Because the data are available on CD-ROM and on the Internet, ''an undergraduate can sit down at a computer terminal and test an idea he got the night before while standing at the checkout counter.''
The map was compiled from radar-altimeters flown aboard the US Navy's $80-million Geosat satellite and the European Space Agency's ERS-1. The altimeters measure the distance between the sea surface and the center of the earth to within a centimeter's accuracy. Because the gravitational effects of undersea formations can cause the nearby water to bunch up against them, these objects leave a tell-tale topography on the sea surface that, while undetectable to the human eye, can be gleaned by radar.
The first inkling that satellites could be used this way came during the Seasat program in the 1970s. Although the mission was designed to study ocean waves and currents, researchers found they could correlate data on changes in sea surface with subsurface features. ''This whetted the appetite of marine geophysicists,'' McNutt says. Unfortunately, the satellite only survived three months into its one-year mission. Moreover, its orbital ground track left 100-kilometer (60-mile) gaps between passes.
In 1985, the Navy launched Geosat, whose orbit gave its ground-track gaps of only 10 to 15 kilometers, dramatically increasing the resolution of the undersea topography. The satellite gathered data for four-and-a-half years until its tape recorder gave out. Pressure to declassify the Geosat gravity-field data came with the launch of ERS-1 in 1991. It would achieve similar resolution, but its data was not classified.
Shortly after Geosat was launched, the Navy agreed not to classify gravity-field data below latitude 60 degrees south, near Antarctica. In 1992, the declassified data crept north to include everything below latitude 30 degrees south. All Geosat data was declassified on July 19. The new map was completed in September.
''This is allowing us to have our cake and eat it too,'' McNutt says. ''We now have global coverage and high resolution.''