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Porth yr Ogof – Wales’ Largest Cave Entrance

Step into Wales’ largest cave entrance, where the Afon Mellte vanishes underground in the heart of the Brecon Beacons’ Waterfall Country.

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Porth yr Ogof, near Ystradfellte in the Brecon Beacons National Park, is home to Wales’ largest cave entrance, where the Afon Mellte river vanishes underground. The imposing 20m-wide portal opens into a maze of limestone passages, underground streams and waterfalls, making it a classic training ground for cavers. While the entrance chamber is accessible to visitors in dry weather, the full system is a serious caving destination, best explored with experienced guides. The surrounding Waterfall Country offers scenic walks and dramatic gorges.

A brief summary to Porth yr Ogof

  • Aberdare, Ystradfellte, CF44 9JF, GB
  • Visit website
  • Duration: 0.5 to 3 hours
  • Budget
  • Environment icon Outdoor
  • Mobile reception: 3 out of 5
  • Monday 12 am-12 am
  • Tuesday 12 am-12 am
  • Wednesday 12 am-12 am
  • Thursday 12 am-12 am
  • Friday 12 am-12 am
  • Saturday 12 am-12 am
  • Sunday 12 am-12 am

Local tips

  • Check river levels and weather before visiting; the cave can flood rapidly after rain, making it extremely dangerous.
  • Wear sturdy, grippy footwear – the path down and the cave floor are rocky and slippery, especially when wet.
  • Bring a torch if you plan to go inside the entrance chamber; daylight fades quickly, and the floor is uneven.
  • Heed all warning signs – the resurgence pool is deceptively deep and fast-flowing; only experienced cavers with proper gear should attempt to cross it.
  • Combine your visit with the nearby Four Falls Trail or other Waterfall Country walks for a full day in the Brecon Beacons.
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Getting There

  • Car

    From the A465 Heads of the Valleys Road, follow signs for the Waterfalls Information Centre near Pontneddfechan, then continue towards Ystradfellte to the pay-and-display car park at Cwm Porth. The drive from Pontneddfechan takes about 15–20 minutes on narrow, winding roads; parking costs a few pounds for several hours.

  • Public Transport + Walk

    Take a train to Neath or Merthyr Tydfil, then a local bus to Pontneddfechan or Ystradfellte. From there, it’s a 3–4 km walk along country roads and paths to the Cwm Porth car park and cave entrance, taking about 45–60 minutes on foot.

  • Guided Caving Tour

    Several outdoor centres in the Brecon Beacons offer guided caving trips into Porth yr Ogof, including equipment and expert leadership. These typically last half a day and cost around 40–60 GBP per person, suitable for beginners with some fitness.

  • Walking from Ystradfellte

    From the village of Ystradfellte, follow footpaths and country lanes for about 3 km to the Cwm Porth car park. The walk takes roughly 45 minutes on mixed terrain, with some steep sections and uneven surfaces.

Porth yr Ogof location weather suitability

  • Weather icon Any Weather
  • Weather icon Rain / Wet Weather
  • Weather icon Clear Skies
  • Weather icon Mild Temperatures

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Discover more about Porth yr Ogof

Gateway to the Underground

Standing before Porth yr Ogof, you face what is widely regarded as the largest cave entrance in Wales: a vast, arched opening nearly 20 metres wide and several metres high, carved by the Afon Mellte through Carboniferous limestone. The river’s name, meaning 'lightning', hints at its nature – a flashy, rain-fed stream that can transform from a gentle flow to a raging torrent in hours. In normal conditions, a dry riverbed leads into the day-lit entrance chamber, where daylight still reaches deep into the cave, but after heavy rain, water floods the passage, making it impassable and extremely dangerous.

A Cave for Cavers

The full Porth yr Ogof system extends over two kilometres with multiple entrances and a complex network of passages, streams, sumps and waterfalls. It is no longer a commercial showcave but is instead used as a training and introduction cave for caving clubs, schools and outdoor centres. Key features include the Wormhole, Letterbox and Creek passages, as well as the Inner Sanctum and Waterfall Series, all requiring proper caving gear, experience and often a guide. The resurgence pool, where the river re-emerges, is particularly hazardous and has been the site of several tragic incidents, underscoring that this is not a casual tourist cave beyond the entrance zone.

Geology and Landscape

Porth yr Ogof lies in the heart of Waterfall Country, where rivers draining the Old Red Sandstone hills cut steep gorges through bands of Carboniferous limestone. The cave is a classic karst feature, formed by the dissolution of limestone by slightly acidic water over millennia. The area is protected as a Site of Special Scientific Interest and Special Area of Conservation, recognised for its geological and ecological value. The surrounding woodland and gorges, with their ferns, mosses and waterfalls, create a magical, almost primeval atmosphere that has even drawn film crews looking for otherworldly locations.

Visiting the Entrance

For most visitors, the experience is about standing in the immense entrance chamber, feeling the cool, damp air, hearing the river’s rush and seeing how the landscape literally disappears underground. A short, steep descent from the Cwm Porth car park and visitor area leads down to the river and the cave mouth. In dry weather, it is possible to walk a short way inside with a torch, but there is no formal path, and the floor is rocky, slippery and uneven. Warning notices clearly state that only properly equipped and experienced cavers should venture further, and the resurgence pool should never be crossed without specialist training and equipment.

Access and Safety

The cave is managed by the Brecon Beacons National Park Authority, with a pay-and-display car park and basic visitor facilities nearby. Parking is available at Cwm Porth, close to the Waterfalls Information Centre in Pontneddfechan, and the site is well signed from the A465 and A4059 roads. While the entrance is open year-round, conditions change rapidly with the weather; what is a safe walk one day can become a life-threatening situation after rain. Visitors are strongly advised to check river levels and weather forecasts, wear sturdy footwear, and never enter the cave alone or without proper preparation if planning any exploration beyond the entrance.

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