The Grotto's organic shapes are eerie after dark, and strange in the daytime, too.

The Grotto

San Antonio, Texas

On the west bank of the San Antonio River, where it cuts at an angle through the heart of downtown, stands a towering rocky outcropping of weird natural shapes. But it is oh-so unnatural. It isn't even rock.

What could possibly go wrong down here?

It's The Grotto, created in 2009 under the direction Carlos Cortes, the Texas master of trabajo rustico (rustic work). Cortes's father was a student of Dionicio Rodriguez, who'd created the similarly carbuncled Crystal Shrine Grotto in Memphis, Tennessee. Although Cortes had aspired to become a doctor, he instead followed his dad into this unusual line of work.

The Grotto is built out of steel-reinforced concrete, hand-shaped while wet by Cortes and his team, then stained to resemble weathered wood and rock. Cortes calls the resulting artwork, when it's done right, "perfect imperfection." The tools that are used are simple, such as kitchen cutlery and old house-painting brushes, but the messy technique requires a degree of blind faith and a lot of speed, because once the concrete dries, it's too late. The Grotto is so big that it took 12 fast-working sculptors nearly a year to complete.

Artist Carlos Cortes at work in 2009.

A winding staircase from the Camden Street bridge descends to The Grotto through the skull-like fanged open mouth of a giant concrete Mesoamerican jaguar -- an early indicator that The Grotto is not normal, although Cortes has said that some tourists don't really notice it.

Once at the level of the river, winding passageways through the fake eroded rock are embedded with counterfeit fossils of prehistoric sea creatures. Carved tree roots from a giant concrete stump can be seen snaking along the cave-like walls. Simulated cavern stalactites hang from the ceiling, fake stalagmites rise from the floor. Far above, next to the stump, an elongated upright loop atop The Grotto mimics the holy sunburst shape around Our Lady of Guadalupe, a familiar religious icon in this part of Texas (If Mary has in fact visited The Grotto, Cortes isn't saying).

Tree-mushroom thing signals the approach to The Grotto.

On days when the temperature tops 100 degrees in San Antonio, the subterranean shade provided by The Grotto makes it a popular outdoor spot. The Grotto is such a good pseudo-cave that it's become a habitat for bats.

Some of the organic shapes visible in The Grotto are recognizably terrestrial, others are unsettlingly alien, although even the familiar ones can be strange, such as a bearded "Father Nature" who spits a waterfall out of his mouth. A close study of the walls reveals human faces, although Cortes has said that not all of them were intentionally put there.

San Antonio's busy Riverwalk passes through the lower, open-sided part of The Grotto, and tourists on river barges slowly motor past, staring at the other tourists in The Grotto. Even after dark the Grotto tourists are on display, as the entire riverfront facade is lit at night.

Seashell-sprouting stalactites: nature gone haywire.

Cortes earned a lot of money building The Grotto, then spent a year in prison for failing to pay taxes on it. Rather than ruin his reputation, his absence only increased the demand for his work. He remains the go-to guy in Texas for those who want something made of concrete that resembles a tree or a cave.