This is a really interesting story – and more will follow, no doubt, as research continues. How much is genuine geology and how much is speculation, or computer simulation, remains to be seen. The Curiosity Rover will eventually resolve some of the issues. The story has been around for several days and has now percolated to www.dailygalaxy.com/my_weblog/2012/12/ancient-cavern-systems-of-mars-pot… … via some weirding of the story. Gary sent the link to www.marsdaily.com/reports/Vast_Systems_of_Ancient_Caverns_on_Mars_May_Ha… … and it seems an extensive system of caves has been found on the edge of the Hebrus Valles, an outflow channel that extends some 250km from two zones of surface collapse. Expect this story to grow legs.