The mountains of the Brecon Beacons are made of Old Red Sandstone, laid down in the Devonian period when this part of the world was a river delta near the equator. The southerly dip of the rock strata gives the range its distinctive profile: gentle slopes rising from the south, ending in dramatic north-facing escarpments that drop away into glacial cirques carved during the ice ages. The Welsh word for these round-headed valleys is cwm, one of the few Welsh words that English has borrowed wholesale. Cwm Sere, Cwm Cynwyn, Cwm Oergwm, and Cwm Cwareli line the northeastern face of the central ridge like scoops taken from the mountain by a vast hand. Beneath the sandstone, the geology shifts. The limestone country at the park's southern edge holds the Ogof Ffynnon Ddu cave system and the 90-foot Henrhyd Falls, where water plunges into a gorge that featured as the entrance to the Batcave in a recent film.

Bronze Age burial cairns crown the summits of Pen y Fan and Corn Du, placed there by people who climbed these mountains four thousand years ago to honour their dead. One cairn on Fan Brycheiniog was excavated in the early 2000s and dated to approximately 2000 BCE; a wreath of meadowsweet had been placed with the cremated remains. Over twenty Iron Age hillforts dot the park, the largest being the twin forts of y Garn Goch near the village of Bethlehem in Carmarthenshire. The Romans stationed more than six hundred soldiers at Y Gaer near Brecon. Norman castles followed, including the striking Carreg Cennen, perched on a limestone cliff. And between these eras, Welsh drovers walked their cattle and geese to market in England along tracks that still cross the high ground, bringing back gorse seed to sow for their sheep.

In February 2013, the entire national park was designated an International Dark Sky Reserve, one of only a handful in the world. On a clear night, away from the towns in the valleys, the Milky Way arcs across the sky with a clarity that urban dwellers have largely forgotten is possible. The designation reflects the park's remoteness from major light pollution but also the deliberate effort of communities within it to shield their lighting and preserve the darkness. The western half of the park gained separate recognition in 2005 as the Fforest Fawr Geopark, acknowledging the geological significance of the Black Mountain range and the landscapes shaped by the Caledonian and Variscan mountain-building episodes that folded and faulted these rocks hundreds of millions of years ago.

The Brecon Beacons have a second reputation, one earned in suffering. The Special Air Service and Special Boat Service use the mountains to test the fitness and endurance of applicants, and the 'Fan dance,' a gruelling march over Pen y Fan carrying heavy equipment, is a rite of passage for those seeking to join Britain's elite military units. The mountains are not forgiving. In July 2013, three soldiers died from heatstroke during an SAS selection exercise on the range. Earlier that year, an army captain was found dead on Corn Du after training in freezing conditions. The park draws 4.4 million visitors a year, most of whom come for gentler pursuits: walking the ridgelines, photographing the waterfalls near Ystradfellte, or watching red kites and peregrine falcons hunt above the moorland where Welsh mountain ponies graze.