Graffiti has been sprayed on the rocks at the protected Dwejra Heritage Park, an act of vandalism which has infuriated visitors.
The spray - a section of which bears Thursday's date - has defaced what is considered a haven for geology and ecology enthusiasts in Gozo.
Regular visitors to the area said graffiti was first sprayed earlier in the month and the vandalism appears to have spread in recent days.
The rock chamber itself was probably used before the tower on site was built in 1652 to guard the Fungus Rock.
"The graffiti has encouraged others to continue defacing site," nature walks guide Annalise Falzon told Times of Malta.
"The area was once meant to be a Unesco site - such acts continue undermining the conservation needed on site and accentuates the total lack of enforcement. Ironically, there was much more enforcement at the time of the knights than nowadays - even if now we know much more about the precious nature of the site in terms of archaeology, ecology, geomorphology..."