- [Elisha] The first mudslide happened, and then about 30 minutes later, the second one happened, and then the road, it went down.

- [Reporter] And Elisha Richardson's house became a kind of island in the middle of flowing mud, water, and debris.

- We were trying to keep an eye out for tornadoes, so we were actually out here when the first one happened, and it just sounded like a rolling rumble as it was just coming down, just like a bunch of dirt just kind of tumbling down the mountainside, almost like an avalanche.

But with your mud and everything, it definitely put off a sound.

I was thinking, wow, this is insane.

I would say the only time I was scared was when my husband's out here and the second one happened, and he almost got, it compromised him.

I was just like, oh my god.

He almost got hurt.

- [Reporter] Landslides scarred North Carolina's mountains.

It's as if a giant monster scraped the hillsides with its claws.

The US Geological Service reports more than 1500 landslides in North Carolina associated with Hurricane Helene and the heavy rains that fell for several days before the storm arrived.

Some landslides are large, some small.

Landslides ate away at the fill dirt beneath road beds, washing out hundreds of roads.

Engineers are assessing the damage.

That repair bill will run into the billions of dollars.

- My first impression, it's going to take some type of retaining wall and reinforce it and get this thing back safed up 'cause the guardrails gotta be removed, shoulder rebuilt, and then the guardrail reinstalled.

- [Reporter] Landslides took out businesses as well.

Hoyt Johnson's mountainside hotel, the Big Lynn Lodge, was spared, so he opened the doors as a shelter for storm victims and first responders.

- People can come see what I call the new mountains, the new look.

- [Reporter] I was gonna say, what do you mean, you said that a couple times, the new look, the new mountains.

- Yeah, 'cause it is definitely different, where there used to be mountains, it's washed away.

The creeks run different than they used to.

Where businesses unfortunately are gone.

So, it's definitely new mountains, it will take years to fix.

- [Reporter] The images are terrifying.

So what happened?

First, let's define what happened.

- So the term would be, for out in Western North Carolina, would be mud flows, if it's really rich in fine grain materials, silts, and clays, or probably a better term is debris flows, 'cause there's boulders and trees and mud and everything in between that's coming down.

- [Reporter] Debris flows or landslides require two things, gravity, that's obvious, but they also need a slope that is steep and long.

- But once you get up in the Upper Piedmont and then into the Blue Ridge, now we have slopes that some of 'em are over a kilometer long or 5,000' long.

There's lots of vertical relief between the top of the ridge crest and the valley bottoms.

Those slopes are steep.

They're at or near what we would call threshold hill slopes, so they're at about, essentially as steep as you can get without material moving down 'em.

And it rains tens and tens of inches within a short period of time on soils that were already fairly wet because it'd been raining in the preceding days, then this is what we would expect from the landscapes of the Southern Appalachians.

Water's percolating through the soil on the hill slope, and then it intersects the bedrock underneath that soil column, which is maybe three to 6' thick.

And then, the water can no longer penetrate into the rock, so then it starts flowing along the rock soil interface, and the soil column loses coherence, and then that whole mass starts to slide down.

If it's oriented towards a channel, it slides down into the channel, and then that's where you transition from a landslide into a debris flow.

Typically, the debris flows will start up high, near the ridge crest.

The soil prism itself fails, starts to slide down into a channel where there's water flowing, then it bulks up.

And that channel already has a lot of debris in it, rocks and wood.

Once the landslide enters that channel, it combines with the water and the debris that's in that channel already, and then that starts snowballing essentially and morphs into a debris flow that then is traveling at 30 to 60 miles an hour.

But we're not surprised that this type of hazard happens in the mountains, it's what we would expect, and it's why North Carolina Geological Survey has a group that's tasked with identifying locations where landslides have happened in the past, and then thinking about where are other places, where they may have not triggered in the last rain event, but where are other places that have similar characteristics, in terms of slope, steepness, soil thickness, certain rock types that may be favorable to generating landslides and debris flows, and then generating these maps that can then be used by county or state emergency management folks.

- We just have to be prepared for what is here and what may come.

We all can't run to a different place.

There are ties, there are generational homes, there's a real sense of community in a lot of these places.

So we are accepting a certain amount of risk when we live in these places, but we can be smarter about how we live with this risk.

- So, we knew it was gonna flood, we just didn't know how much.

- [Reporter] Over in Old Fort, the Mountain Gateway Museum remains standing despite Mill Creek washing the debris from nearby landslides through the town.

The 1930s home that houses the museum was built during the depression, out of rocks taken from the river.

- [Roann] We've been lucky with this rock building, it was well built.

- [Reporter] But she admits there is a risk to living in a mountain area she loves.

- When Mother Nature gets upset, she's gonna destroy whatever she decides to destroy, no matter how well it's built or whatever safeguards you take.