Strange underground polygons on Mars hint at Red Planet's wet past 

 Buried dozens of meters under the equator of Mars is a large honeycomb pattern similar to, but much larger than, those found only near Earth's frigid poles. 

https://cdn.mos.cms.futurecdn.net/jxfvYGygPFydCNFqusCxc5.jpg

https://www.space.com/mars-underground-polygons-evidence-ancient-water-red-planet