[Roaring, rushing noises] People: Whoa. Whoa. That's bad. [Announcer] It's Sunday, and the side of a mountain has just collapsed. It crashes into the river that runs through the Rhondda Var, near Tylorstown. Nobody is injured, but it's only today the sheer scale of that collapse is obvious. Something else that is clear is the amount of coal that was still on this mountainside. Gerald watched it all happen. [Gerald Durham] There was a roar, and you could see the muck starting to slip from the top area, there. Within a couple of hours, the whole lot had come down there. [Announcer] Do you feel vulnerable? [Gerald] Yeah. Yeah. With this going on, yeah. These valleys are surrounded by coal. They were built on coal. It's a frightening prospect, you know. If that could've happened on this side of the mountain, it would be a totally different outcome. [Announcer] His father Cliff is a former miner. You look at this landslide now, what goes through your mind? [Cliff Durham] Oh, Aberfan, straight away. Believe me, we don't want to see that again. I wouldn't. [Announcer] I've been speaking to people on this street. It's Bryn Heulog, and it's opposite the landslide. They described yesterday as eerie. The sound of slurry crashing down the mountain. They are just thankful that this happened in front of their homes, not behind them. The greenery of the valleys disguises the amount of coal that is still hidden here. In the 1970s, 80s and 90s, the land was reclaimed. But now, there are calls by local people, to inspect all the areas where coal was dumped. [Chris Bryant] We should definitely be inspecting all the former coal tips. Making sure that if there is excessive water in there, that that is being drained off, make sure everything is safe. Because obviously, people have seen the horrific pictures on television news, of what's happened in Pontypridd. People might not have seen it for themselves. It is much bigger than it looked on television. So, people are, of course, worried. It is one thing, having water coming, coursing through your house. It is quite another, having, um, slurry, and the top of the mountain coming down through your house. [Announcer] But, who is responsible? Who owns the land? The Coal Authority says, it's not them. The Councils say, they're not sure if they do. Who is responsible for these sites, and who is maintaining them? [Andrew Morgan] At present, we are in discussion with Natural Resources, Wales. Clears on the side of the hill. It is the big one. Rhondda Var is a concern to us. We have engineers on site right now, monitoring it. [Announcer] Do you think that is your land, or Natural Resources Wales' land? [Mr. Morgan] We are still in discussion over that. Um, but at present, we have engineers on site, trying to understand if there is a risk to the properties, or the public. Certainly there is some concern about the moment that happened yesterday, and what's caused it. Is it just water on the surface, or is there a deeper issue within the mountain? That is currently under investigation. [Rushing noises.] [Announcer] This is what happened after a weekend of heavy rain. These valleys know all too well of the histories with landslides. Now, some are questioning whether this was a one-off, or an early warning. Rob Osborne, ITV News.