Ingo
Below are some photos of Coco Crabs in their burrows. These were generally inside hollowed out tree stumps near the 7 site. We did a bit of exploration in the crab burrows - remove crab, and while a crab wrangler entertained said crab, willing and brave subject crawled top half of torso into crab burrow - to see if there was anything interesting to be found in the burrow. Nothing of interest was found, which would validate your finding that the burrow is really a cool place to rest, not a place to drag food or shiny things home to.
These burrows were not underground as we've heard about, and may not be exactly equivalent to what is the norm, but these crabs seemed happy, at least until I stuck my camera in their faces.
Andrew