M

From the Super Mario Wiki

M's are strange marks in Super Mario Sunshine, that, when cleaned, produce a Blue Coin, and, on one special occasion, one regular Coin. They are all around Isle Delfino. They do not appear out of the island. They can appear on the wall, on the sides, and even on the ground where Mario is standing. There are also M's that are invisible; Mario must shoot water at them, and they will appear.

[edit] Trivia

• The M's resemble an english M with an umlaut above it. An umlaut is a diacritic consisting of two dots placed above a letter.