When the Prometheus enters the LV-223 atmosphere, the crew starts to look for a place to land.
The planet was big. They would have to have incredible luck to go into the atmosphere where actually the structure was, or at least very close to them.
How is such luck possible? Is there any more explanation to it?
Basically, they were following natural/artificial terrain cues that the Engineers also likely followed or left behind:
Obviously this sequence has been abbreviated for dramatic effect. In reality, they most likely would have spent an hour or two trying to map the planet from high orbit, and, if not, they probably would have circled the planet a few times below the cloud layer to find the most promising landing spot.
But when you watch the sequence, it's not hard to imagine that the Engineers probably also encountered the lone mountain peak jutting through the clouds, decided to enter at that point, and then simply followed the natural contours of the terrain to arrive at their canyon protected building site.
In the grand scheme of things, this sort of "luck" isn't that hard to explain. It's similar to how wildlife pathways form in nature. There aren't signs or surveys or detailed planning by the wildlife. But the characteristics of the terrain encourage certain travel/migration paths.
The film also mistakenly stated that LV-223 was about as distant from Earth as Jupiter rather than in a distant star system, because Damon Lindelof doesn't know mathematics. There is no explanation in the film for how or why the crew chooses the landing site that they do. It's just poor writing, which is to be expected in Lindelof's work.
There may well have been an explanation in the original Jon Spaihts script, as many of Prometheus's more baffling plot-holes were explained in that script, but I unfortunately do not recall. It might still be online, somewhere, though when I last looked it had been taken down. It's worth a look, if you can find it.
