Then, the Company Information menu option would have its own navigational system going into each company the conglomerate owned, and so on. As a possible option, when sites get this large, it can be good to think of each sub-navigation piece as its own site. For example, our conglomerate owns six other companies. The Company Information link would have a menu option for Companies. This link would simply go to a page listing each company's name and a link to that company's Web site.
Site maps are often a good idea.
A site map is usually an HTML version of the flowchart you developed in the planning stage. A little fancied up to look nice, but essentially that is what it is. I like to offer users a site map as an alternative way to navigate a site. In my experience, they are very handy and successful. As Web browser software matures, so does the technology you can use to manipulate the browser.