Let's just be up front about this now -- the Titans left stuff everywhere. It would be hard to disbelieve in them, frankly. They left bases, research stations, fortresses, labs and more. We don't even know what everything they left behind originally did or why it was there in many cases. Some places have somewhat clear reasons for existing (Ulduar, for instance, was tasked with holding the Old God Yogg Saron prisoner, but the Halls of Stone and Lightning point to other goals for the complex) but others, such as the ruined complex now known as Ahn'Qiraj was simply a 'research facility', and we have no idea what it was researching or why such a complex was needed so close to Uldum.
At any rate, there are a lot of Titan complexes currently known of on Azeroth.