It is estimated that nearly 32 million people have visited Mesa Verde National Park since it was created by presidential proclamation in 1908.

And for over 100 years nearly every one of those visitors have flocked to the park's crown jewel - Cliff Palace, the largest cliff dwelling in North America.

But on Thursday, the park's jewel was closed off, visible only from an overlook as archaeologists started to work on preserving the site, repairing extensive cracks discovered a few years ago.

Heavy foot traffic and gravity itself - and the fact that the dwelling is built on the side of a steep alcove - have all taken their toll over the years.

"Everything is slowly moving downward," said Gary W. Ethridge, a preservation archaeologist with Mesa Verde National Park said, as he pointed to a heavily traveled path on the edge of an alcove.

Cliff Palace was first discovered in 1888 when ranchers Richard Wetherill and Charlie Mason combed the canyons for their cattle. It boasts 150 rooms and 23 kivas.

Since its discovery, the cliff dwelling has attracted many in awe of the construction and lives of the ancestral Puebloans who occupied the area an estimated 700 to 800 years ago.

"A lot of the frontal architecture is not very well supported," said Ethridge. "In addition, this site is heavily visited. There is a picture of the site in the '70s, where there is so many people standing on the site, there is not a spot anyone else could stand."

Archaeologists plan to address that path and retaining wall first, because if it slides, the rest of the site can be in jeopardy, because everything is built in tiers.

Up inside the site, Ethridge pointed to a well masoned kiva below us - Kiva F. The round ceremonial room currently has large cracks running down the side of the wall, and one wall is bowing inward. Currently, the kiva is supported by a makeshift support constructed with two-by-fours - hardly authentic.

"We no longer let folks walk around Kiva F," Ethridge said. "We did the best we could to support that wall, but we need to do better."

During preservation, Kiva F will likely undergo some reconstruction, and it may also be a candidate to be filled.

Filling the structure with dirt up to a certain point will give the structure support in some areas, yet allow visitors to see part of the kiva structure.

"We won't likely fill it up all the way. Plus, there are other, very well preserved kivas here," Ethridge said.

Currently, archaeologists are doing test excavations under the lower trail so that they can eventually put supports under the trail.

A crew of about 10 plan to work on repairing cracks throughout the 800-plus-year old structures.

"Everything is settling and slowly moving and cracking inward," Ethridge said.

The crew will use a mobile hoist to drop down supplies over he alcove's lip from the top parking lot.

Crews also plan to do some preservation work at the very top of the alcove, where rooms seem to hang on a precarious ledge, that is actually an arch inside the alcove.

"Those rooms are remarkable," Ethridge said. "They are all original construction."

But preservationists need to work on some of them to ensure they stay on the ledge.

"We just want to do a bit of stabilization and need to keep people from walking under it," Ethridge said.

This isn't the first time archaeologists have battled to preserve Cliff Palace.

Building the parking lot above the site's alcove cause unique problems with water drainage and impacted the natural filtering the sandstone has and also cause water to runoff and damage other areas that water historically didn't run off into.

To combat moisture problems, a tunnel was built in the '60s to keep water out of the site and in 2012 water runoff from the parking lot was piped out and away from the site, Ethridge said.

Archaeologists also plan repair damage caused by a 1995 water line break that flooded some of the site.

This spring and fall, tourists who come to see Cliff Palace will have to visit other sites in the park as park archaeologists and preservationists work on the site. Cliff Palace will still be visible from the overlook.

Preservation will halt on the Thursday before Memorial Day, May 25, and the site will reopen to limited tours for the busy tourist season and close again for more preservation work after Labor Day. Archaeologists will plan to use a unique masonry mixture to do most of the work. Preservation started in December.

"We are really just fighting the forces of nature," Ethridge said.