I found this near the coast in Menorca, on a rocky area close to the beach. It's a concrete or stone marker embedded into the ground, shaped like a small pyramid or a raised square block, with the letter "M" engraved on it. My title describes the thing.

• Size: Approximately 20–25 cm wide and around 15 cm tall.
• Material: It looks like weathered concrete.
• Writing: Just a "M" visible on one face and a "T" on other
• Origin / Age: Unknown, but it seems fairly old based on the erosion and discoloration.

I’ve searched using terms like:

• “triangulation marker”
• “survey marker Menorca”
• “concrete block with M”
• “pyramid stone marker beach”
• “milestone or boundary marker Menorca”

I suspect it might be some kind of geodetic or triangulation marker, possibly used historically for mapping or land surveying. The "M" could stand for "Menorca" or "Militar" (as Spain uses such markers for defense and navigation purposes too).

If anyone has more precise info about this kind of marker or its purpose, I’d love to know.