In the US, national parks are most at west side. The top three big national parks are #1: Yellow Stone National Park, established in 1872, about 136 years, #2: Yosemite National Park, established in 1890, #3: Grand Canyon National Park, established in 1919, about 90 years.
Before 1870, west side was an unexplored area. Therefore, congress send explorers to discover it. There are total of 32 people in the team, included all different kinds of professions, arrived at Wyoming in 1870, and spend 1 year to explore and did the report. The report included details with handwriting and photos, then got back to congress in 1871. Congress was impressed by how amazing the west side was. Therefore, in 1872, the first national park was established.