The giant squid, Architeuthis dux, evaded being photographed alive for the whole of the 20th century. Finally in January 2002 one was caught by chance off Japan and photographed; since then there have been half a dozen giant squid caught alive and photographed.
Here is a slideshow of all known living Architeuthis photographs.
No-one truly knows how large this grows, the largest male caught measured 10m in length, but it’s thought the females grow even larger…
Cuttlefish camouflage: a quantitative study of patterning: here.