Hi everybody,
By comparing three pictures taken by the Mars Express spacecraft for Dena, and two for Wendy, I discovered that these caverns were in fact only big holes in the martian ground. You can see everything on my blog : http://marscope.blogspot.com, or on http://marscope.blogspot.com/2007/09/domma...es-de-mars.html.
Sorry, it's in french...