Mars rovers have uncovered a variety of different things on the red planet, including geographical features such as dried up lake beds, patches of salt crystals believed to be former sites of salt water bodies and multiple soil samples that were analyzed on site. There have been several rovers sent to Mars since the first attempt in the 1970s, though only a few of these rovers, including the United States Spirit, Opportunity and Curiosity rovers, have actually been able to successfully collect useful data.
Both Opportunity and Curiosity were still operational as of February 2015, with each rover frequently taking pictures of the landscape around them and sending that information back to Earth. In addition to collecting data about geography and geology, Mars rovers have been monitoring the red planet's atmosphere.
For example, thanks to the rovers' work, scientists now know that the gaseous element argon is present on Mars. These rovers have also taught scientists a lot about how weather works on Mars. It regularly experiences weather phenomena such as dust devils, which indicates that the red planet is windy in spite of its relatively thin atmosphere. The rovers have also found plenty of evidence of ancient water sources on the planet.