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Behind miles of chain link fence on the Texas State University campus in San Marcos, the remnants of the Aquarena Springs Resort sits crumbling. The old turnstiles that used to admit hundreds of thousands of people annually are still visible beneath years of overgrown weeds, but the gondola trams and observation tower are long gone.

In its heyday in the 1960s, '70s, and '80s, Aquarena Springs was one of the most popular tourist destinations in the state of Texas. People came from all over the world to see underwater mermaid performances and a swimming pig named "Ralph."

"It's a piece of history. That whole era is gone," said longtime San Marcos resident and photographer Andy Heatwole, peering into the remains of the park.

"Not only will this not come back, but you're not going to have anything like it," he said, aiming his camera at what is left of a replica Spanish mission.

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Aquarena Springs also offered a resort hotel — built in 1929 — and a submarine theater. But today, one attraction remains: the glass-bottom boat tours that skim the clear water of Spring Lake every half hour.

"There's a whole lot of action going on under this water," said Jerilyn Dutton, who brought her family from Westminster, Colorado for a boat ride.

"Look at that fish!" said her daughter Addison, pointing through the glass pane into the blue deep.

"The glass bottom boats are one of the most significant educational tools we have," said Andrew Sansom, the Executive Director of the Meadows Center of Water and the Environment at Texas State University. He has overseen Aquarena's most recent transition into a nature center.

Even without the pigs or mermaids, Sansom says the Meadows Center is still a "globally significant site." The San Marcos springs are home to eight federally listed or endangered species.

As modern theme parks came into their own, kitschy Aquarena struggled to compete with their speedy roller coasters and high-tech animal performances.

"In today's terms, it was kind of like a mom-and-pop carnival. I mean, it was funky!" said Sansom. "But it was wonderful. I mean it was a family-oriented deal and you could get in a submarine and go to the bottom of the lake and watch a show."

But for those who visited the park at its peak, it will never be the same.

"I photograph it because it's a piece of my history," said Heatwole, who came as a kid. "It was a really neat place, it just couldn't compete."

Jason Hunter, a photojournalist with WISN in Milwaukee, contributed to this report. This story was originally reported for the NPPA Advanced Storytelling Workshop in San Marcos, Texas.