And it's a subject that's close to my heart, because it's about my hometown.

I was born and raised in Halifax, Nova Scotia and as we film this, we're coming up to the hundredth anniversary

of one of the most major events in the history of that city. But it's also an event that was marked around the world.

Because it was the largest explosion made by human beings in the Pre-Atomic Era.

Key to understanding this story is understanding the geography of Halifax as a city.

And so I'll sketch it out here. The main part of Halifax is a peninsula.

Quite a funny shape. In the wider area,

and what made Halifax such an incredible resource, for hundreds of years is...

the larger picture and the harbor and how it opens up into the Atlantic Ocean.

So you have, next to this peninsula, this deep and sheltered basin called the Bedford Basin.

And then, you have this tiny little passage called the Narrows, with a little bump here.

And then this beautiful deep Harbor. You've got a few islands here, you've got a big ones here, McNabs Island.

You've got a little one here.

Out here is the Atlantic Ocean.

Over here is the Bedford Basin.

This side is a city, the city called Halifax. And on the other side is a city that used to be called Dartmouth.

Now they're put together in one Regional Municipality.

But we'll call them separate sides to keep everything in track. Key to this story is this narrow little passage here.

Which is actually called the Narrows. Let me set the scene for you: 100 years ago, it's

1917 December, World War 1 is fully in force and this is an incredibly strategic point.

Hundreds of ships could fit in this nice protected Basin.

And then be shepherded over the North Atlantic in convoy to Europe. It's the morning of December 6, 1917.

Halifax is waking up. It's a very busy town. It's got a big naval presence. It's got a big army presence.

So the night before, every night in fact, submarine nets would have come down across the harbor.

To protect all the ships in this part of the basin from U-boat attack.

Submarine nets would have gone down across here and across here and limited the traffic in and out of the harbor.

And so we now have to introduce two boats to our story.

Number 1 is the [SS] Imo, this is a Norwegian registered Belgian relief ship.

So this was a neutral ship that was tasked with carrying supplies

to give relief to civilians on the continent that had been devastated by war.

So this is sitting here in the basin. It's been sitting there for a while and it's getting impatient.

It's meant to be leaving on the 5th, but by the night of the 5th,

it still hasn't been loaded up with coal and the submarine nets are down.

It's stuck, it wants to go to New York. Coming in from New York, is the [SS] Mont-Blanc.

And this is a French ship and unbeknownst to almost anyone in Halifax. It's carrying. It's loaded.

It's packed to the brim with explosives.

So it's also intending to go across the North Atlantic and help with the war effort.

It arrives too late

to get through the submarine nets, so it spends the night over here, next to McNabs Island.

What happens the next morning, these two ships both want to get through the harbor in different directions.

According to the "rules of the road", they should be passing port side to port side.

Which means they should come across like this (right lane moving) navigating carefully and slowly to get through the Narrows.

However, that's not what happens.

The Imo takes this corner wide to avoid an oncoming ship (passing on the left).

While the Mont-Blanc is keeping its station on the correct side.

At this point, there are a few kilometers apart, and remember,

it's 1917 and the conventional way of communicating is through whistles.

So the Mont-Blanc whistles first, that means by rights,

they should retain right-of-way. And they say one whistle, "I'm staying where I am"

The Imo, unbelievably,

responds with a cross signal, two signals whistles, "No, I'm keeping this station." Now we have a problem. The Imo was moving quite fast.

It's not loaded with anything, so it's sitting high in the water.

And it's moving probably faster than the speed limit. The Mont-Blanc cuts its speed.

Again, one whistle...

Even moves over slightly to the starboard side, the Dartmouth side. "I'm staying here." Two whistles from the Imo.

"No, I'm staying here." At this point, heads would have turned. The first exchange,

people are used to that background noise. The second exchange,

people would have known that something unusual was happening. And would have stopped to look. And at that point,

it's quite clear that these two ships are

heading straight towards each other in this narrow channel, with very little room left to maneuver. Remember how nervous

the captain of the Mont-Blanc must be, he knows what's in his cargo. Cuts his engines, says:

"Ok, I've got to do something to mitigate this situation," cuts his engines, which causes the ship to drift towards the Halifax side.

Had that been the only thing that happened,

the situation would have been avoided. It would have been a near miss, but at the same time, the Imo

also cuts its engines. And because it's sitting so high in the water, its propeller has really little purchase and it has very little maneuverability.

And so now a collision is absolutely inevitable. And so, the Imo impacts the Mont-Blanc broadside (right),

like this.

Even then,

a fairly minor collision in the grand scheme of things. What makes this story really interesting, is to understand fully,

what exactly was on the Mont-Blanc. We have a variety of things, all designed to blow up.

We have 227,000 kilograms of TNT.

Have 1.6 million kilograms of wet picric acid, as well as,

544,000 kilograms of dry picric acid. On top of that, 56,000 kilograms of gun cotton and

strapped in barrels to the top of the ship, we had

223,000 kilograms of benzol and monochlorobenzol.

So let's talk about what these things actually are, the benzol and the monochlorobenzol,

they're petroleum byproducts and they're used, sort of, in the production of picric acid, which let's talk about a minute.

They're flammable. They are in danger of catching a fire.

What's packed in the holds, however, is something more dangerous.

Now TNT at that time, was

becoming to be used in the production of munitions. It was largely replacing picric acid as the explosive powder

used in munitions, because picric acid is so incredibly volatile and dangerous.

So while the new TNT was perhaps less powerful as an explosive, it was

relatively speaking, much easier and safer to handle than the picric acid. We've now got our two ships, then they sort of are

maneuvered apart. Okay?

So they're now separate. The Mont-Blanc

drifting towards the Halifax side. In the initial collision, a fire started on deck. Now it may have been from the sparks.

Caused by the metal hulls grating against each other. It was likely caused by the the simple,

you know detonation, of a few grains of picric acid.

But once the fire took hold, all of these flammable things on the top deck

started to catch fire. And so now, you've got a ship on fire. [Brady] This is mainly the benzol. [Dr. Meghan Gray] This is mainly the benzol, yeah.

It's spewing off thick oily black smoke, and people are starting to become worried, because the ship is drifting towards the Halifax side.

It's heading straight for some of the wooden piers that are the heart of the harbour business. Okay?

So they're worried that there's gonna be a fire on shore, they're worried about pollution in the harbor from oil.

That would be bad enough. On the ship,

the crew immediately know that there is absolutely nothing that they can do to stop this ship from blowing up. I should mention that,

in sensible non-war times, if you were carrying a ship like that, you would be flying flags all over the place. Saying...

You know, "Danger, stay away." For obvious reasons,

you don't want to do that in war-time. So very few people actually knew that it was a ship loaded with high munitions.

The crew immediately,

seeing how futile the situation was, got into their lifeboats. They start moving towards the Dartmouth side.

They're shouting all the way, there shouting warnings to people. But the warnings are either not heard or they're not

understood because they're in French. So now we've got the situation where the ship is on fire,

it's now nestled against pier six on the Halifax side. The fire is raging,

the benzol and then later the monochlorobenzol

is reaching boiling points. Those barrels are shooting up into the air and exploding.

So it's a big spectacle, people are rushing to windows. They're rushing down the streets to see what's going on.

What's happening in the hold,

it that these are hermetically, they're carefully constructed holds. To hold these high explosives.

The fire is raging, the temperatures are increasing, and whether eventually it's from the incredibly high

temperatures and pressures inside these sealed compartments, or whether it's an impact from these exploding barrels coming down and hitting the deck.

Detonation occurs: the picric acid

explodes, sends a shock wave through the neighboring barrels, the hold explodes, sends an air blast the rest of the ship.

One by one, the holds full of dry picric acid.

The wet picric acid and the gun cotton, which have the water inside them evaporated,

turning them into high explosives as well.

Detonates as well.

You've now got a ship with three kilotons of high explosives,

which is now the largest bomb that human beings have ever made and at 09:04 on December 6th, 1917,

it exploded.

Well instantly, there is a massive fireball that reaches 5000 degrees Celsius.

That's nearly the temperature of the surface of the Sun.

And it engulfs the neighboring streets.

[Brady] But people on the land... [Dr. Meghan Gray] The people on the land, incinerated.

The Mont-Blanc, three million kilograms of iron hull,

destroyed.

White-hot shards of iron are flown through the air. Faster than bullets and rained down on the neighboring area.

Most devastating was the air blast.

Traveling a 1000 metres per second, through the narrow streets. It causes

unimaginable devastation to houses, to human beings, to the whole whole city.

Remember that many of these people are standing looking at this spectacle.

Lining the steep hills, leading down to the harbor, looking out at the burning ship.

When it explodes, all of the windows in the city exploded as well. And most of them are driven inwards.

Horrifically injuring people. Everything that wasn't immediately blown up as a structure, catches on fire.

It's a huge firestorm throughout the city. As the air blast

goes out, and the gas is cool. The air rushes back in again and

tornados full of debris, start sweeping through the streets. The 1.4 thousand kilogram

anchor of the Mont Blanc was flown

kilometres away. Way over on the

other side of the city and can still be seen today where it landed.

Going back to what happened in the harbor, the expanding gases push the water away,

forming a hole in the in the water in the harbor.

Just like when you throw something into a pond, and and you see the ripples rebound,

caused a huge geyser to appear in the harbor.

That sucked water away from the opposite shores and then sent a huge tsunami

funneled down these narrow harbor walls, reaching 20 feet in height, causing even more devastation.

At the end of that morning, you had 2,000 people dead,

9,000 people severely injured,

12,000 homes destroyed and

every single

building within a 12 mile radius damaged.

[Brady] You can find out more about the Halifax Explosion.

Including some links in the video description, and I'll also have details about more from this interview with Dr. Gray.

Also thanks to Neil Barnes from our chemistry channel, Periodic Videos for helping out with some of these demonstrations.

And you can find out more about Periodic Videos, also in the video description.