JOPLIN, Missouri - Joplin's EF-5 tornado is the costliest singe tornado in U.S. history, causing nearly $3 billion dollars in damage.

A lot was lost, but a lot was also learned, including how to build a better, more storm-resistant hospital for the future.

It was hard to put into words how widespread the damage of the May 22, 2011, Joplin tornado was. Mile after mile there was nothing but destruction, including the hospital.

Now, four years later, Mercy has built a new $465 million hospital, and its new tornado-proof construction is getting nationwide attention.

The tornado left 161 people dead and 7,000 homes leveled or damaged beyond repair.

Many thought the safest place in Joplin during the tornado would be St. John hospital, but with winds of up to 250 miles-per-hour, even it wasn't spared.

5/23/2011 Related Story: Deadly Joplin Tornado Sheds Light On Hospital Emergency Plans

Mercy's Executive Director for Strategic Projects, John Farnen said, "I could never have believed that that type of substantial building could actually sustain that kind of damage."

Hospital employees, like Chief Nursing Officer Dennis Manley, will never forget the images.

5/20/2012 Related Story: Hospital Workers Share Harrowing Accounts Of Joplin Tornado

"The first thing I noticed was all of the windows, with the curtains blowing out of the windows," he said.

Farnen said, "Just being in awe saying ‘Holy Cow, what do we do?'"

Mercy later faced the question of what to do long term, since the old hospital couldn't be saved.

"We do a lot of disaster planning, but not necessarily planning to lose everything," said Farnen.

They wanted a new hospital to be able to withstand a tornado, and 39 months later the new 900,000 square-foot facility is open.

The entire exterior is made of reinforced concrete panels.

During the tornado, the hospital's roof was ripped to shreds, turning materials into projectiles. The multi-layered roof on the new facility is much stronger.

"If you can protect the shell of the building, to prevent the damage from coming inside, then you have a good chance of keeping everyone inside safe," Manley said.

Mercy also worked with a manufacturer to create specially designed windows. In the lobbies, they're rated for 110 mile-per-hour wind. The strongest ones are in the ICU, and can withstand 250 mile-per-hour wind. They're also covered in a plastic laminate film.

"That glass will not break up into shards and kind of fly around," Farnen said.

Inside, there are hurricane-rated doors, each floor has a fortified safe zone with reinforced walls and ceilings and stairwells and critical life support systems have battery backups.

The new hospital also has crucial supplies stashed away in places on every floor in the facility, like shovels and crowbars, in case they're needed to clear debris from hallways.

Farnen said, "In the old facility, all of our emergency supplies were in the basement."

Above ground, the new hospital is impressive. Below ground there's an impressive investment in another part of the project.

Underground, 20 feet underground, a tunnel runs one and a half football fields, and connects the hospital to its physical plant. The bunker protects all critical utilities, like electric lines and communications, which were all wiped out in the 2011 tornado.

"We actually have two sources of normal power, one from the north and south. So if we actually get hit from either side, we still have all the power we need from the other direction," said Farnen.

The tornado-proof construction added $11 million to the project.

Mercy is looking at implementing the fortified tornado construction into other facilities in tornado-prone areas.

Hospitals in Oklahoma, and nationwide, are also looking at the construction techniques used in Joplin, according to Farnen.

"After the event here and Moore, Oklahoma, and we had another facility hit in Oklahoma in Edmond, so definitely everybody is looking at this and taking it more serious," he said.

The reinforced construction gives hospital employees more peace of mind and increases the chances of the facility being able to operate if another powerful and violent tornado hits again.

"A time like that is when they need the hospital the most," Farnen said.

The new hospital has duplicate systems for just about everything, including generators. It's also able to use four different cell phone carriers, has a direct link to emergency management, and there's a HAM radio operating center.

The disaster-resilient elements of Mercy's design only increased its price tag by three percent.

The Cherokee Nation also sent a team to look at the hospital construction in Joplin to see what can be implemented in the tribes' new and existing hospitals and clinics.