City Hall staffers found approximately 50 mining-era sites on acreage owned by the municipality as part of a survey required of the largest landowners in Park City as leaders continue their efforts to reduce the danger of the relics.

The results of the survey were submitted early in the fall. Other landowners that fall under the regulation must turn in their evaluations and mitigation plans by Dec. 1. The date was outlined in a law enacted in 2011 requiring landowners with 10 acres or more inside the Park City limits to search for mining-era sites and then eliminate any hazards that are found.

Heinrich Deters, who is the trails and open space project manager at City Hall, said none of the approximately 50 sites found on municipal land were deemed a hazard that needed to be mitigated. He said 38 City Hall-owned tracts of land were surveyed. Deters said it took staffers 40 hours to visit the sites in June and August. He estimates hiking 25 miles through the municipal land searching for sites.

"We had lots of evidence of the old mining shafts . . . None of them were a hazard, per say," Deters said.

He described many of them being depressions in the ground or what appeared to once be entries into mines. He said mining sites that would have been exploratory in nature were also found. Some of the sites are known as adits in mining terminology.

Deters said staffers researched historic documents and maps as they prepared to survey the acreage. Purchase documents, leases, business records and aerial maps were consulted, he said. One of the maps, made by the U.S. Geological Survey, dates to 1955, he said.

"We were so surprised at the accuracy of that old mining report. It was astonishing," he said.

Deters said staffers plan to provide the information to Mayor Dana Williams and the Park City Council at a meeting on Nov. 29.

The Dec. 1 deadline applies to each of the between 40 and 50 landowners under the regulation. Jim Blankenau, the environmental programs regulatory manager at City Hall, said he had received most of the surveys by Thursday. He had not reviewed them by then.

Landowners may ask for a one-year extension. Officials would consider granting one if the landowner was making progress. Blankenau said none of the landowners had requested an extension by Thursday.

City Hall provided a form for the landowners to complete, asking whether someone visually inspected mining-era sites like shafts or tailings and whether they made a reasonable effort to learn of the sites through researching documents.

Once the sites that need to be made safe have been identified, the landowners have until Dec. 1, 2015 to complete the work.

The law was enacted in 2011 and was spurred by a series of highly publicized incidents involving mining-era sites that occurred over several years. In one case, the top of a mine tunnel gave way as someone skied over. Snow cascaded into the tunnel and the skier had to make his way out of the depression that was created in the snow. In another episode, a dog tumbled down a historic mine shaft. A firefighter rescued the dog.

Park City was founded as a silver-mining camp in the 19th century and the mining industry drove the economy until the middle of the 20th century. Silver prices sank, though, and the ski industry eventually was started.

Mining-era relics remain scattered through the Park City area. Some of the larger ones have become iconic locales in the community, but officials worry about the smaller ones that are not known and could pose a danger. The law defines a hazard as an opening that extends five feet or more into the ground. The opening may either be vertical or horizontal.