One of my favourite dive locations is the Lembeh Straits (North Sulawesi). Easy to get to from Singapore and one of the top "Muck" locations in South East Asia. In Lembeh there is always a surprise around the corner, under a rock, buried in the sand or hiding inside an empty bottle. On one of the dives during a macro photography workshop a couple of years ago my guide found this Octopus using two empty halves of a coconut shell as his hiding place. I took around 12 shots saving my film as I was hoping to find something more interesting later on. When I saw the developed film the next day I was pleasantly surprised by the results.
Ever since I have been looking for this sort of behaviour. It's amazing to see what an octopus can use to create his hiding place. I've seen them use glass jars, discarded crisps packets, halved drinks cans, empty bivalve shells and bits of bamboo. To me this is one of the things that keeps diving and underwater photography interesting.