- Landslides are often a sudden, terrifying collision of weather and geology that challenges the very words we use to describe stability.

Grounded, rooted bedrock.

It's almost impossible to predict their exact location, time and size, but we do know a lot about the conditions that lead to them.

Landslides generally occur when the natural structures holding a slope in place become weaker than the force of gravity pushing down on that slope.

And they're often triggered by a destabilizing event like heavy rain, an earthquake, volcanic eruption or wildfire.

Human activity like logging or a leaky swimming pool at the top of a hill is also another common cause.

On average they cause around $4 billion of damage and kill dozens of people in the U.S. each year.

Landslides also occur in every U.S. state and mountainous areas around the world.

And the danger they pose is on the rise as we continue to move into hazardous areas and our warming climate makes events that trigger them, like fires and extreme precipitation, more frequent.

One such event that led to the most expensive landslide in recent U.S. history was the 280,000 acre Thomas Fire in 2017.

Which, at the time, was California's largest wildfire.

- [Marco] Thank you.

Thank you guys.

Thank you.

- [Maiya] First, the fire left the mountains above Santa Barbara and Montecito barren of the vegetation and roots that normally hold their soil in place.

- [Marco] This is my first real view of the mountains.

They are just naked.

They're naked.

Nothing left up there.

- [Maiya] Then on January nine 2018 a storm swept across the mountains, dumping a half inch of rain in a five to 10 minute burst on the steep slopes above Montecito.

Marco Farrell was outside the landslide evacuation zone and went out to check on the neighborhood just after the downpour.

- The flash flood's right there.

Get outta here, go!

Oh my God mom.

Close the door!

Get ready to go out.

Wake Dad up.

I ran in with probably a second and a half to spare.

This is literally as high as the kitchen counters.

And God I pray for everyone here.

- In all, 130 homes were completely destroyed.

23 people died and many more were injured.

This type of event is worth understanding because landslides tend to happen repeatedly in the same locations.

In Montecito's case, they experienced what's known as a debris flow.

These differ from subsurface landslides because, rather than failing as one mass, debris flows originate from a saturated slurry of fine grain sediment.

And then they grow in volume as they travel downhill, picking up mud, rocks and even boulders and trees along the way.

- We are headed up to the top of the Santa Ynes mountains to take a look at the source area for the Montecito debris flows.

- [Maiya] Towns are often built below steep, fire prone areas like these because of the natural beauty they create.

But this also presents a real risk.

The rule of thumb is that if a slope is steep enough that walking up to its peak is difficult, then, with enough rain, a slide becomes possible.

- If you were standing right here during the rainstorm you would see water running off the surface from here down the hill slopes and producing these small channels that we call rills.

And so you would see thousands and thousands of little mud rivers going into the channel and all that mud coalescing in the bottom of channels and starting to flow and becoming progressively deeper as you go from the top of the watershed down to the bottom.

Within that five to 10 minutes we already had a couple of thousand cubic meters of mud and that mud was able to pick up boulders.

And by the time that you reach the bottom of the mountain you've got a 40 foot tall wall of boulders and trees.

With boulders the size of SUVs and even larger.

And this 40 foot high wall of rocks is traveling at 20 to 30 miles an hour.

And as you exit the mountain front, you go out into the fan.

It can then spread out into a lower gradient area where we build our houses and those 40 ton boulders that are 10 feet across and 10 feet high are able to just rip through a house like it was a hot knife through butter.

- As shocking as this is, it's nothing new to the area.

Here on the Southern coast of California a thousand years can pass between events this large so we forget.

Then years later, we build communities in the direct path of landslides.

And it turns out that these flat areas at the base of the mountains where towns like Montecito and Santa Barbara sit were actually created by previous landslides.

But with such long intervals between these massive slides and no cultural memory of similar disasters, convincing citizens to evacuate for rainstorms can be very challenging.

Pat McElroy was the Santa Barbara fire chief.

In the month before the landslide, he'd helped with multiple evacuations as the Thomas Fire threatened the area.

- This is one of the few places where you can still see what it looked like two and a half years ago.

And unfortunately in this neighborhood four people lost their lives.

You know, hundreds of homes were destroyed.

Hundreds of lives were turned upside down and there's still people dealing with the impacts of that.

We'd kept people out of their homes for quite a bit of time, right around Christmas.

So there was a certain amount of evacuation fatigue that was in the community.

- Debris flows are different from most of the disasters we're covering in the series because there isn't much you can do to prepare and protect your house.

So evacuating is key.

But many communities just don't have evacuation plans specific to this type of hazard or detailed information needed to create them.

Before the Montecito event, authorities decided on an evacuation line that mirrored the Thomas Fire evacuation, which ran horizontal to the coast along Highway 192.

But as debris flows barrel down creek beds too small for the massive volume they overflowed and poured on the streets and in the homes of people who did not expect to be impacted.

- In this case what we learn is the evacuation models that we'd used for fire weren't effective in a debris flow.

- So the vast majority of lives that were lost were in the voluntary zone, not the mandatory zone.

- And now that we've learned this community has a history of these types of events going back hundreds of years, if not millenniums, we have to have this in our planning process.

How would we evacuate differently?

- [Maiya] Updating evacuation plans for debris flows could save lives in Montecito and far beyond.

And the community is implementing another solution that could help lessen the impact.

- Two years ago, that stuff came uninterrupted from the top of the ridge to here.

And so what if you can slow that momentum by a series of nets?

So when it intersects with people and houses again, it doesn't have quite the force, doesn't have quite the momentum.

So we're in San Ysidro Canyon above Montecito.

And this is where a lot of the major debris flow activity occurred.

This is actually the first net we put in.

It's a braking system, it's like a shock absorber.

The rings would allow fine soils and water to go through.

And as the material started building up behind it the net loads, then those break rings will fire off and they deform until they're straight.

And what that is doing is transferring that added shock into those anchors that are sunk deep into the sides of the creek.

Are the nets we've built big enough to stop what happened on January 9th?

No.

But will they make an impact?

Yes.

- When it comes to any type of landslide knowledge is the single most important factor in keeping yourself safe.

Remember that a combination of water and steep slopes is the perfect recipe for most landslides.

Keep an eye out for signs of past slides and indicators of slope instability such as tilting trees or cracking ground.

If you have any concerns that your home may be at risk from one of these disasters, get in touch with local officials or a qualified geologist to dig deeper.

Sometimes the only warning that a landslide is about to occur is a few falling rocks just before a much larger area gives way.

So pay attention, make a plan and remember the words of Montecito resident, Marco Farrell.

- It's a real pain in the butt to evacuate but it could save your life and it could also save the life of a first responder that has to go and try to find you and rescue you.

- What I've learned from Montecito is everything can get rebuilt and it's getting rebuilt.

If there's a debris flow risk in your area, evacuate, leave your house behind.