Measuring  8m long by 2m tall, this rare geological marvel was discovered by Madrid-based mineralogists in the abandoned Mina Rica, where iron, lead and silver were mined until the Spanish Civil War. Guided tours lead visitors 60m underground down corridors and metal steps, culminating with a chance to clamber inside the geode and view its dazzling collection of translucent gypsum crystals.

Tours leave half-hourly throughout the day and must be booked in advance via the website. http://www.geodapulpi.es/