Assuming it doesn't eat your group first, of course.

Scientists had long considered the Giant Squid, architeuthis dux, the most massive invertebrate in the sea. Very little is known about them, as none have ever been seen alive in the deep ocean; all we have are corpses and scars on on the Sperm Whales who eat them. Based on the sucker diameters of some of those scars, there have been estimates of squid out there up to 100 feet long at full stretch.

We now know that these Colossal Squid do indeed exist, that they're another creature entirely, that mesonychoteuthis hamiltoni is not only larger but better equipped to defend itself, and that they venture to the surface on occasion because that's where this specimen was when caught near Antarctica. I've seen live squid; they're ridiculously quick (see video) and agile. Hamiltoni is absolutely a creature you do NOT want to meet up close and personal. This incident may not be the first time one has been seen on the surface, either. If that older account is correct, even the size reported by the BBC understates the case.

Our world is not yet fully explored, and science still brings us wonders to behold.

     
UPDATE: In our post-modern age of non-privileged viewpoints, please remember that one man's biological wonder is another superbeing's eldritch minion. The Ctulhoid "Tentacles of God" website refers to this incident as: "Human scientists desecrate the body of another shallow water explorer." Winds of Change.NET recommends you show a little sensitivity, and perhaps ask yourself "why do they sacrifice us?" Could incidents like the one described above be perpetuating the cycle of violence?

(Squid graphics courtesy of TONMO.com.)