Where in the country can you freely roam around?

Sure there's a few nature trails here and there, but where, if anywhere, are you free to just go around? There's so much bare and/or uninhabitable land out there with nothing but nature, but it feels like everywhere is private property. It seems like there's no exploring, no vast swathes of land that you're allowed to go around in. Is there some sort of map that shows what land is open to the public? It seems even most public nature reserves are limited to designated walking areas and trails.