Giant squid, colossal octopi and sea monsters - these mythical beasts capture our imagination, often appearing on the silver screen wrecking boats and terrorising people. Just look at how we portray these creatures in Pirates of the Caribbean or Clash of the Titans. But apart from being in our minds, do such 'monsters' exist? In the dark depths of the ocean, do we really have ancient beasts the size of oil tankers that can wreak havoc at whim, with eyes as big as saucers and tentacles the length of buses? The answer is yes, no and maybe. Yes, in the sense that we really have animals bearing resemblance to what we see in films and read about in literature. Giant squid and octopi do exist - but they're not of oil-tanker size. At most they can grow about 15 metres long, or in exceptional cases up to 20 metres. So, really, the answer should be no - because we don't see beasts of epic proportions emerging from the water and destroying everything in their path. Even if they may seem other-worldly, they have natural predators; adult sperm whales are known to pick fights with giant squid and try to eat them. Suddenly, these animals of the sea don't seem monstrous any more. But maybe ... because scientists just don't know. The fact that we've never seen such creatures means we can't confirm their existence - just like aliens. Furthermore, not only are tales and lore of sea monsters heavily ingrained in cultures all around the world, there is also evidence to suggest that somewhere in the depths, these beasts do swim. Take a look at legends such as the Loch Ness Monster in Scotland. The evidence is inconclusive, but at one point even the British government officially believed in its existence and deployed police to ensure it would be protected from poachers. Some old sailor hand or old ship logbook might tell of legends passed on down the years, of overturned boats with all hands lost ... Perhaps the Lusca - reportedly a giant octopus in the Caribbean that some say grows up to 60 metres long - is the closest to fictional images. The only specimen ever recorded was from an ancient carcass washed ashore, which had rotted so much that only ambiguous conclusions could be drawn. No matter what, the hunt continues, and the sea monsters that live in our imagination still elude us in the deep.