Perhaps it's mentioned and discussed many times before, anyhow.
There are lots of sites discussnig the tubes of mars (e.g. http://ida.wr.usgs.gov/fullres/divid.../m0400291a.jpg )
I'm not gonna talk about underground irrigation systems, or submerged ufo-rollercoasters.
Could it be that these things are ICE, since they do look 'glassy'? If water, assuming there would be enough water underground, turns into ice as soon it hits the Martian surface, could it than be that the tubes are remainings of water-'geisers', that changed into ice tubes over time?
If so, it could also be a place to find running water inside, wouldnt it?