I don't recall when I first heard of hollow earth as a bona fide theory (so I'm not counting stories like Journey to the Center of the Earth by Jules Verne, which I read as a teenager), but I think it was on the website/forum LDSAVOW. Someone started a thread about it, and the idea made sense to me. That was easily 20 or more years ago.
It was here on MindReach that the idea was brought up that earth isn't the only hollow planet.
Scientists seem to hate anything that goes against their traditional beliefs (just read Graham Hancock's books to see that), so I don't expect any serious exploration in this area.
I just looked at the thread about Mars, and the purple color at one end. To me, that gives credence to the idea that it's hollow.
I wish scientists were willing to look at all science and nature in the manner in which they pretend to study things - with an open mind, being completely willing to be wrong, and no one loses funding or jobs if they discover something that goes against the status quo.