A crop of "blueberries" found by Mars' Victoria Crater by a NASA rover may indicate ground, not surface, water altered the area, U.S. space officials said.

NASA officials said they thought the "blueberries" were formed by iron-rich minerals when groundwater percolated through sediment. The spheres were scattered across the surface almost everywhere the Opportunity rover trekked from its landing site to the crater.

The crater's rim was "full of great big, juicy blueberries," the rover's chief scientist Steven Squyres said during the American Geophysical Union's meeting in San Francisco. "That was a surprise to us."

Because the "berries" form underground as groundwater rises through the soil, the finding suggests the water rose to a certain level then stopped without reaching the surface, Squyres said.

The finding was unexpected because Opportunity earlier found ripples in the nearby Endurance Crater -- indicating that water occasionally flowed on the surface, NewScientist.com said.

Just how far deep the water level is won't be known until Opportunity descends into the crater in a few months.

"As we go down, we'll cross a bathtub ring," marking the highest level the water reached, Squyres said.