This long path is the first area in the route to The Paddraean Archaeopolis. You will find the mark for Mission 04 here, as well as the Cie'th stone for Mission 05. More importantly, you'll find the path to The Deepgap, the next area you should visit to get to the Archaeopolis. After you've visited the Archaeopolis, there will be an optional cutscene if you return to this area.