The island M stands for Mauritius
Scientists have discovered a submerged continent in the Indian ocean, between Madagascar and India. According to sediments found on the coast of Mauritius, at some point during the last 2 billion and 600 million years ago, there was an archipelago that separated from Madagascar and the Indian sub-continent. They then got submerged during the tectonic plate movements that resulted in the way land masses exist today.
Reference: Torsvik, T. H. et al. Nature Geosci. 2013, 223. http://dx.doi.org/10.1038/NGEO1736
Further reading: Sid Perkins in Nature News
Image credit: Nature Geoscience (Supplementary information)
Update: A reader pointed out that perhaps this was the origin of the legends of Lemuria. Although that is not accurate, the Wikipedia article on Lemuria is worth a read.