A Sicilian immigrant almost a century ago hand-carved rooms and passageways in the hardpan beneath the thin farmland soil, and used it as a living space, a cool refuge from the hot California summer sun. Down there he also planted several fruit trees that could receive sunlight through skylight-type openings above. Fruit trees underground, people! It's my Underworld! It's even designed after the ancient Roman catacombs, so, properly Mediterranean.
Amusing addendum: my mom lived in Fresno in her teen years, and found out about this place when she was out with some friends one night. The guys said to the girls, "We're going to show you this cool place, but you have to tell NO ONE." At the time, the Underground Gardens were just fenced-off territory with "no trespassing" signs around it, so they had to sneak in with flashlights. Apparently most people in the city had no idea it was out there; you can't see much from ground level.
So my mom was late getting home after exploring the place, and her parents demanded to know where she'd been and why she hadn't called. (This was the 1950s, well before cell phones.) She finally broke down and told them about it, begging them not to get anyone in trouble. Her dad (my grandfather) declared, "Daughter, I sell real estate insurance. I know every square foot of this area, and I know there is no such place. Where were you REALLY?"
And that forever remained his final word on it. He never believed her. (He died more than 20 years ago.) But now you can tour the gardens, which really do exist! :)