Oranges, grapefruits, lemons, limes—the half-dozen or so types of citrus found in most grocery stores are just a small fraction of all the wild and wonderful varieties in the world. More than 1,000 of these can be found at the University of California Riverside Citrus Variety Collection, a.k.a. the Noah’s Ark of citrus.

The groves house two trees of each species, used for research, plant breeding and more. The diverse collection includes some wonderful and unusual varieties of citrus: yellow- and green-striped lemons, football-sized pomelos, and heart-shaped grapefruit with deep red veins striating its flesh. Some varieties, when cut, ooze a mucosal slime, others have tiny juice bubbles with a caviar-like pop.

This episode of Gastropod takes a tour of the unique collection with its director, Tracy Kahn, to dive into the secret history and science of citrus.