Sparkling hollows filled with lustrous crystals, dubbed geodes, are often perceived as small objects that can fit comfortably on a bookshelf. But some are more like gigantic cathedrals filled with a forest of glassy towers.

The Pulpí Geode, found within an abandoned silver mine in 1999 in Spain’s Almería Province, is one of the world’s largest. It's a cavity about 390 cubic feet in volume, with walls adorned with imposing gypsum crystals up to nearly seven feet long. Because of the impressive dimensions of this temple of transparent spires, scientists have long ached to know how it was forged.

As they report this month in the journal Geology, Juan Manuel García-Ruiz at the University of Granada and his colleagues