LAKE OF THE OZARKS -- For nearly 80 years it has been holding back the 600 plus billion gallons of water that make up the Lake of the Ozarks.

Workers began work on Bagnell Dam in 1929 and finished the structure two years later in 1931. Despite its age, engineers say the structure is safe and sound.

"You would think in 1931 they didn't have as good of engineering practices as we have today,” Bagnell Dam Engineer Alan Sullivan said. “What we've found is they did a great job of overdesigning not only the structure itself, but all the equipment inside.  And we have some equipment still operating in the condition that it was in 1931."

Bagnell Dam was built in the midst of the great depression and three-quarters of a century later the owner of the dam, Ameren UE, has installed warning sirens to alert those down river of a dam emergency.

But dams have failed, even another one owned by Ameren UE. The Taum Sauk Reservoir collapse in 2005 destroyed most of Johnson's Shut-Ins State Park and injured a family of five.

So what makes Bagnell Dam so strong?

There are five main reasons national experts say dams fail.  Bagnell Dam is put to the test.

The first reason: structural failure. Faulty construction material can cause structural failure and while there are cracks in the surface concrete, Sullivan said there is nothing to worry about.

“Our exploratory borings, when we've actually drilled into the dam, have revealed very, very solid concrete inside the dam,” Sullivan said. “Plus they put a lot of reinforcing steel in the dam. So there is not a structural deficiency because of the design of that era."

Also, engineers said Bagnell's design is the preferred type of concrete dam, a "gravity dam."  It's shaped like a right triangle with the base resting on bedrock.

Ameren said it has since improved upon the initial construction.

"In the 1980s, we drilled over 300 holes in the dam and installed a post-tension anchoring system,” Sullivan said. “Those are actually nine inch steel rods that go down into the bedrock below the dam and actually tie the dam into bedrock."

While Ameren touts that anchoring system, a study presented to the Association of Dam Safety officials criticizes the method used in Bagnell Dam for not using corrosion protection. The study says that inspections of a dam in California that used the same anchoring method showed that the steel rods had corroded giving the structure no added strength.

Sullivan said that they are aware of that issue and are exploring a way to inspect Bagnell's steel rods without damaging the structure.

Structural failure can also occur from an earthquake. But Sullivan said that Bagnell should survive a quake coming from the most likely source: the new Madrid Fault in the Bootheel. Sullivan cites a study that showed fractures in the earth's surface between the dam and the fault will keep the full strength of a quake from reaching the dam.

Another reason a dam can fail: overtopping. That's what happened at Taum Sauk. A computer software problem caused the reservoir to continue filling above its normal level. A failure at the Truman Dam, nearly 100 miles upstream, could also overtop the dam. But engineers at Bagnell said that the anchoring system installed in the 80's should protect against any overtopping failure.

Dams have also failed after inadequate maintenance and upkeep by its operators. Sullivan said different inspections are done on a daily, weekly and quarterly basis at Bagnell Dam. Ameren also does more in-depth checks every year and again once every five years.

Another likely cause for a dam failure: cracking caused by movement, like the natural settling of a structure. Sullivan said that's not a problem at Bagnell Dam.

"We also do a movement survey so we can tell if the dam, with survey instruments, we can tell if the dam is moving at all,” Sullivan said. “And we are not seeing significant movement of the dam, just seasonal expansion and contraction.”

The fifth a dam likely fails: piping. When a leak in the dam collects sediment, it forms sink holes in the dam's interior. Sullivan points out that scenario cannot happen at Bagnell Dam because it is a concrete dam, not an earth-filled dam.