This is probably globigerina limestone, commonly used in Maltese buildings. The limestone is formed mainly out of planktonic foraminifera. The weathering pattern you see is alveolar weathering due to the proximity of the sea: salt crystals form inside the limestone's pores, which causes the integrity of the stone to deteriorate. This leaves the stone more vulnerable to other forms of weathering. An important factor in creating this type of weathering seems to be an heterogenous wind flow over the stone's surface, but the precise cause of the honeycomb appearence has not yet (as far as I know) been found.

Best wishes,

Kat (Letters and Comments editor, New Scientist)

I had always thought these cavities were formed by wind erosion, aided by sand etc (associating them with deserts and windy coastlines). I am pleased to have now learnt that they are indeed caused by chemical processes involving salt.

I am clearly no expert on this subject, but was interested to find out that they are called "tafoni", and that these features have also been observed on Mars!  The following website may be of interest to the person asking the question.

www.tafoni.com