The Sakari civilization was decimated by an alien attack that wiped out the majority of their population. They did not know who the aliens were, but Voyager's first officer, Commander Chakotay, discovered evidence of their identity: the Borg.
The surviving Sakari retreated into the caves beneath their planet and set up systems to try and hide their existence in case of future attacks. This was only partially successful and the crew of the Voyager helped them improve the process in exchange for gallicite. During this encounter, the Sakari demonstrated an impressive ability to camouflage themselves against their surroundings. (VOY: "Blood Fever")