The Lighthouse Ruins are an old ruined lighthouse situated on an island in the Grand Line.
Currently in the first half of the Grand Line.
The lighthouse was originally an old ruin. The nearby island nation converted it into a lighthouse at some point in time in order to guide ships safely into the harbor. However, the island nation gradually shifted it's culture further along the Grand Line, and the lighthouse fell into disrepair, becoming a ruin once more.
Unknown. Some kind of ancient tower. It has been theorized to have once been an ancient observatory of some sort, or perhaps a watchtower.
Currently just a ruin, occasionally ransacked by bandits and pirates.
Naturally, as what happens with all ruins, there are rumors of treasure hidden within the old lighthouse, undiscovered by the people who originally converted the ruins into a lighthouse. Of course, treasure hunters have scoured the place and not found anything.