Fifteen years ago, Opportunity rover started exploring the surface of the Red Planet, originally as a 90-day mission in order to find out what Mars holds. One of the first things it discovered during the first two months was these spherical objects scattered on the surface. It was something that scientists began to refer to as 'blueberries' because they seemed bluish and round. Many theories have been proposed as to what these objects could be and you may read them here. Now, only recently, Opportunity had been retired but the Martian 'blueberries' remains one of its biggest finds as it points to the evidence of possible presence of water on Mars.
(Image credit: NASA/JPL-Caltech/Cornell/USGS/Modesto Junior College)