|1900 brought in telephone service to Two Harbors.|
|In 1902 five Two Harbors businessmen signed the articles of incorporation for a new mining company named Minnesota Mining and Manufacturing (3M). The corundum mine and mill at Crystal Bay went bust several times before the company reformed and relocated to St. Paul.|
|In 1906, the Lake County Court House, which stands to this day, was built. Rural mail delivery in Lake County is established in 1906 as well. Two Harbors officially become a city in 1907. The Duluth & Iron Range Railroad Depot was constructed in 1907 as the railroad continued to expand its operations. Between 1907 and 1915 the wooden ore docks were replaced with the steel ore docks still in use today in Agate Bay. The Carnegie Library in Two Harbors was built in 1909.|
|In response to the disastrous storm of 1905 the Split Rock Lighthouse was constructed and opened in 1910 between Two Harbors and Beaver Bay.|
|Construction began on the International Highway between Duluth and Grand Portage in 1921 . MN 61 follows much of the same route.|
|Along with the new highway the automobile overtook the lake steamers as a mode of transportation and the sinking of the United States and Dominion Transportation steamer America in 1929 is often viewed as the end of passenger service on the great lake.|
|The Alger Smith Logging Company ceased logging operations in 1919. The Alger Smith closed its headquarters in Knife River and the Duluth & Northern Minnesota railroad would literally "pull up the tracks" in 1921.|
As part of FDRs’ relief programs to combat the Great Depression the Civilian Conservation Corp worked throughout Lake County building many of the State Parks and campgrounds.
1938 saw the merger of the Duluth & Iron Range Railroad with the Duluth Missabe and Northern to form the Duluth Missabe and Iron Range Railroad.
|1938-1940 the Works Progress Administration built the new Lake County High School in Two Harbors, it was demolished in 2008.|
World War II saw the tonnages removed from the Iron Range and shipped down lake reach peak levels of 20,000,000 tons from the Missabe Range and 17,000,000 tons from the Vermillion Range.
The Duluth Missabe & Iron Range railroad took delivery of the first Yellowstone Mallet Locomotive in 1941.
In 1944, one of the first HMOs in the United States was created in Lake County to serve railroad employees.
The increase in ore production gave rise to bigger ships that in turn led the Army Corp of Engineers to move the east break wall in Agate Bay in 1948.
By 1938 the invasion of sea lamprey had reached Lake Superior and by the mid 1950s’ had all but destroyed commercial fishing on Lake Superior.
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