OF THE two preceding images, the second has been authenticated (it appears in the photostream of a Flickr user named "BlueBec") and the first, though presumably just as authentic, has yet to be sourced. The EXIF data embedded in the image indicates the photo was taken on April 4, 2007 with an Olympus digital camera and was not subsequently edited.
Coconut crabs (also known as "robber crabs," scientific name Birgus latro) typically grow to about 16 inches in length, though there have been reports of specimens double that size. In any case, it is the world's largest species of land-crab and reputedly lives as long as 50 years.
An inhabitant of islands throughout the Indian and central Pacific oceans, the coconut crab tends to stick close to beaches, though it can't live in water. True to both of its common names, the omnivore's preferred food source is fallen coconuts, but it will accept whatever sustenance is at hand, including such delicacies as might be stolen from a garbage can.
The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent Earth atmosphere, with a weight of up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft 3 in) in length from leg to leg. It is found on islands across the Indian Ocean and parts of the Pacific Ocean as far east as the Gambier Islands, mirroring the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar.
The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomen and stop carrying a shell. Coconut crabs have organs known as "branchiostegal lungs", which are used instead of the vestigial gills for breathing. They cannot swim, and will drown if immersed in water for long. They have developed an acute sense of smell, which has developed convergently with that of insects, and which they use to find potential food sources. Mating occurs on dry land, but the females migrate to the sea to release their fertilised eggs as they hatch. The larvae are planktonic for 3–4 weeks, before settling to the sea floor and entering a gastropod shell. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years.
Adult coconut crabs feed on fruits, nuts, seeds, and the pith of fallen trees, but will eat carrion and other organic matter opportunistically. The species is popularly associated with the coconut, and has been widely reported to climb trees to pick coconuts, which it then opens to eat the flesh. While coconut crabs can climb trees, and can eventually open a coconut collectively, coconuts are not a significant part of their diet. Coconut crabs are hunted wherever they come into contact with people, and are subject to legal protection in some areas. In the absence of precise information, the IUCN lists the species as Data Deficient
(Linnaeus, 1767) 
|Coconut crabs live on most coasts in the blue area; red points are primary and yellow points secondary places of settlement|
Sense of smell
Relationship with human beings
|External identifiers for Birgus latro|
|Encyclopedia of Life||2982586|
|Also found in: Wikispecies, SeaLifeBase|
Now for the "Mystery" part to this: it seems nobody seems quite certain where all these crabs live currently or in the past, and according to rumours they are appearing in other places further inland in East Africa, India and Southern Asia, and in South America. They could well be imported to many of the places but most rumours are unconfirmed. They were supposedly former residents of Southern Arabia, Madagascar, Australia and New Guinea but currently are killed off there. I would not be so certain about that part either. When I was first gathering reports of Cyptozoological creatures I included Coconut crabs because it seemed they were undocumented in many areas. Charles Darwin thought they only lived on one island in the whole Pacific just because he hadn't heard of them being anywhere else.
Well the stories are true they ARE that big. And the stories about them turning up in places like East Africa and South America (Brought in along with the coconuts) could also be true much of the time. The problem is simply that we haven't documented them there yet.