Beneath the surface of Lake Mead, located 35 miles southeast of Las Vegas, Nevada, a world unfolds in shades of teal. Palm-sized bluegill fish nibble the meat from cracked mussels, natural stone spires rise from the depths and shipwrecks slump along the silt bottom. But the lake also hides an array of historic landmarks that only scuba divers can visit: the remains of the massive Depression-era construction project that built the Hoover Dam, including cement tunnels and railroad tracks now decaying in the dark.

In 1931, construction began on what then was called the Boulder Dam. The project sought to tame the erratic Colorado River, prevent flooding in California’s Imperial Valley and generate electricity for the expanding west. Its other purpose was to add jobs: launched by the Bureau of Reclamation during the heart of the Great Depression, the Hoover Dam’s construction employed around 21,000 people and fundamentally changed the surrounding area, building an infrastructure that would lend itself to even more productivity. Train tracks were laid and power lines erected. Boulder City sprung up to house the workers. When the dam was completed five years later, it stood 726ft tall, had claimed at least 96 lives and created Lake Mead, the largest reservoir in the United States.

Today, on summer weekends, the Lake Mead National Recreation Area wakes up early. Marinas buzz with families readying for a day on the water and kids toss bread to massive carp that guard the docks. Meanwhile, the lake fills with speedboats, jet skis, kayaks and even the Desert Princess, a three-level paddleboat that cruises to the dam packed with camera-toting tourists intent on staying dry.

But look closely at the boats out on the water and occasionally you’ll spot a bright red flag with a single diagonal white stripe – the universal sign for scuba divers.

“Lake Mead is considered one of the top freshwater dives in the whole world,” said Bill Duckro, a local scuba instructor. “There’s 700 miles of shoreline. You could dive out there at a different dive site almost every day the rest of your life and not hit them all. And the depths. Even though the lake has dropped more than 130ft, it’s still more than 500ft deep in some places.” After careers as both a police officer and casino wedding chapel minister, Duckro got certified as a scuba diver at age 51 and immediately fell in love with the sport. Today, he owns the Scuba Views dive shop in north Las Vegas, where he leads charters and teaches recreational diver and more advanced certification courses. In a wet suit, he looks like a cross between a nautical Santa Claus and Dog the Bounty Hunter, with the kind of calm, authoritative demeanour that new scuba divers want when their brain kicks into panic mode the first time out.

With Duckro as guide, my fiancé and I headed to Middle Boulder Island for the first of two open water dives in our scuba certification course. We docked just past the Las Vegas Boat Harbor along the rocky shore beneath the concrete shell of a massive tank that juts off the island. A souvenir from the dam’s construction, the tank once cleaned the water that was used to create the Hoover Dam’s concrete.

“We used to hit that tank 80ft underwater,” Duckro said. Today, it’s on dry land.

At its peak in 1998, Lake Mead measured some 1,215ft above sea level, its cool blue-green waters providing a massive break from the surrounding desert scrub. But a persistent drought and decreasing runoff over the last 10-plus years have dropped the water level by more than 100ft. It is now at a historic low, leaving a glaring, white “bathtub ring” of mineral deposit around the shoreline and driving dozens of headlines announcing impending doom for the US Southwest, Nevada and the Las Vegas Valley, all of which depends on the reservoir for fresh water.

Still, as we sank below the surface, there was plenty to see. The tiered ground beneath the tank was busy with fish accustomed to the sight of scuba divers. Duckro grabbed a couple of the invasive Quagga mussels that have infiltrated the lake and held their cracked shells out on an open palm. The fish darted in for timid tastes as we watched, wide-eyed. On the silt floor some 30ft down, train tracks stretched into the murky distance.

Though the dropping water level of the lake is gravely concerning, it does have a faint silver lining for scuba divers: some sites that were once too deep for recreational divers are now within the certification’s limits.

One such site is the Hoover Dam train hopper, where trains would dump loads of rock to be crushed and divided into the aggregate piles. A concrete tunnel runs under the hopper – 8ft high, 10ft wide, 125ft long and totally black inside – a thrilling experience for advanced divers. Recreational divers can explore from outside the tunnel.

Duckro’s other favourite sites include Wishing Well Cove, where wind-sculpted cliff walls narrow dramatically underwater, and the Crack, where natural rock spires rise from the bottom in a beautiful formation. “It’s like you’re going through a maze,” he said of the latter.

The depths of Lake Mead hide dramatic wrecks, too, like the PBY-5a Catalina plane that crashed into the lake in 1949 while attempting a water landing. The pilot and mechanic went down with the aircraft, which now rests a short boat ride east of Boulder Harbor. Even after 60 years in the water “everything is pretty well preserved”, said Steve Fanell, an experienced diver who works in marine salvage and has been exploring Lake Mead for 16 years.

Wreck Alley, a series of boat wrecks just off Sentinel Island, is well-known among the local scuba community. “The biggest wreck there is called the Southern Cross,” Fanell said. “It’s about a 37ft to 38ft wooden sailboat. It’s like the mother ship of all the wrecks.”

But regardless of sailboat size or fame among divers, Lake Mead’s underwater world is still something of a mystery to the people stuck on the surface. It’s a secret kept among a select group of people who dip below the waves.

Correction: A previous version of this article incorrectly stated the depth of Lake Mead. This has been fixed.