What are some of the underwater creatures that have been most dangerous to scuba divers?

 Most divers will agree that you are the most dangerous underwater creature—at least to yourself.

Humans aside, it depends on where you dive. Creatures that actually attack you are quite rare. In over 20 years of diving—over 900 dives in a dozen countries—I’ve never been attacked by anything but jellyfish. And most jellyfish aren’t really dangerous—especially if you always wear a full wetsuit and hood, which I do.

What to worry about is creatures that can defend themselves with deadly force if they think you’re attacking them. These are also few in variety.

The only kill-you fish I know of is the stonefish.

The only kill-you reptile I know of is the sea snake.

Among invertebrates, there’s the blue-ring octopus and a cousin whose name I forget, and cone shells.

Every year some fool diver picks up a cone shell—they can be pretty big as shellfish go, and most are very pretty—and tucks it in his wetsuit to take home to his girlfriend. Instead he gets a Darwin Award. Cone shells have one of the most potent neurotoxins known. No antidote, and usually fatal. Even if you pick one up by the very back of the shell its harpoon is long enough to reach around and nail you.

Why are underwater explosions more deadly than explosions on the surface?

 Imagine you’re relaxing in a swimming pool and suddenly a military grenade drops into the water.

If you think that because water is much denser than air, the shrapnel won’t travel far and you’ll be fine — you’d be right, but not completely.

While the fragments won’t go very far, an underwater explosion has other, much more lethal effects.

Explosives burn extremely fast, so when a grenade detonates, it releases a super-heated gas wave that spreads at incredible speed.
This expanding wave is called a pressure wave — and if it moves fast enough, it can break the sound barrier, creating a shockwave.

On land, a grenade explosion has enough power to tear off limbs, burn flesh, and of course send fragments flying at high speed.
When these shockwaves travel through air and hit a human body, most of the energy is reflected due to the large difference in density.

Only a small portion reaches inside the body, compressing the pockets of gas in your organs — potentially damaging the lungs, ears, blood vessels, and intestines. This internal implosion can cause irreversible injuries.

On the surface, the explosion expands into the atmosphere, which compresses it and reduces its destructive reach.

However, water is considered incompressible — technically it can be compressed, but it requires massive amounts of force to compress even a tiny amount.

This means the surrounding water does not absorb pressure waves as well as air does.

When the pressure wave hits your body underwater, it only slows down slightly because water and the human body have very similar densities — so there is little resistance.

As the wave travels into your body, it instantly compresses the air pockets inside you, causing:

• Brain hemorrhages
• Internal tearing
• Ruptured lungs

If the wave hits the surface and bounces back, the damage becomes even worse due to reflected shockwaves.

So if you ever had to choose between being on the surface or underwater during an explosion…

Your best chance is to run as fast as you can and hope for the best — because underwater, the shockwave is far more deadly.