The Guidestones are a series of monuments in the world of One Piece.
The Guidestones are found all over the world, scattered about the four seas. However, only a few exist in the Grand Line. They're usually erected on tiny islands, sandbars, or rocks jutting out from the ocean.
It's unknown who erected the Guidestones or when. It's suspected that an ancient civilization erected the Guidestones at some point in history when the seas were more manageable.
The Guidestones were most likely originally erected to help guide sailors at sea. Every triangular arrow on a Guidestone points directly to a specific island, with a carving that says the name of the island. In addition to the name of the island, there will usually be a short warning of what to be cautionary of on the way there or when you reach there. Such cautions are usually serious, such as a nest of Sea Kings, or dangerous beasts on the island the arrow is pointing to. Some, however, are somewhat silly, such as one that points to an island and says to be careful not to drink too much ale at the bar there.
The Guidestones are still used to help with navigation, but the changing eras have made that difficult. Many of the islands and Guidestones have shifted slightly over the years, making the arrows not as accurate as they probably once were. Weathering has also weared down some of the Guidestones, making them hard to read. In addition, several islands have sprung up, disappeared, or simply changed names over the years. The Guidestones were probably quite useful when they were first erected. But with modern times, they are of little use other than wonders of the ancient world to behold.
It is worth noting that very few exist in the Grand Line. This is either because the civilization that built them could not easily traverse the Grand Line, or perhaps because the bizarre weather of the Grand Line completely wore away or sunk most of the Guidestones that were erected there.
One of the most famous Guidestones is located near the halfway point of the Grand Line, which points to the Red Line and warns of the Serpent Currents. This suggests that the Serpent Currents existed as far back as ancient times.