Meridiani Planum, the home base of the Mars rover Opportunity, could be the location for a human landing. A recent paper advocated for a landing site somewhere in the vast plain, which Opportunity has explored for the past 13 years. The rover - and its twin, Spirit - landed on Mars in 2004 in search for signs of ancient water, which they've found in abundance.
Water flowed on Mars in the ancient past because its atmosphere was thicker. Over the years, erosion from the sun has thrown away some of the lighter particles, thinning the atmosphere and eventually making it too thin for water to flow on the surface. Atmospheric erosion is a focus of NASA's much newer MAVEN (Mars Atmosphere and Volatile Evolution) mission, which arrived at Mars in 2015.
Meanwhile, Opportunity is still working on the surface under a new two-year extended mission that began in 2016. The rover remains in good working order, although it has had some memory issues in the past couple of years that forced controllers to change how the rover collects and sends data to Earth.
Snake on a Plain