Poole's Cavern is a two-million-year-old natural limestone cave at the southern edge of Buxton, reached by a short walk through Buxton Country Park. The cave was known to the Romans — Roman artefacts and Bronze Age objects have been found inside — and has been open to tourists in some form since at least the 17th century.
The guided tour passes through about 310 metres of passage, through chambers named the Roman Chamber, the Poached Egg Chamber (for the orange-tipped formations), and the Great Dome. The temperature is a constant 7°C, so bring a layer. After the cave, the country park trail continues uphill through Grin Low Wood to Solomon's Temple, the Victorian folly on the hilltop above, adding perhaps 40 minutes to the excursion.