The idea of the attraction goes as far back as 1957, when Walt Disney introduced a concept for an atomic-themed exhibit in Tomorrowland on the Disneyland television show episode, Our Friend The Atom. Plans for the attraction were put on hold for the 1964 New York World's Fair. After the success of the fair, the atomic-themed attraction was green-lighted for the New Tomorrowland project. It would stand where the "20,000 Leagues Under The Sea Exhibit/Monsanto's Hall of Chemistry" building stood.
This was the first ride with an Omnimover system. It opened as a free attraction, unlike most, but required a "C" ticket starting in 1972. The original sponsor was Monsanto, which at the time was an agri-chemical company. Irene Kasmer, using Monsanto fabric, designed the uniforms of the attendants. In 1977, Monsanto ended their sponsorship for the attraction. All visual and audio references were removed (except for the end line, "This is Monsanto") and the Monsanto Mighty Microscope's Monsanto logo was replaced with a logo that says "The Mighty Microscope".
The Monsanto posters advertising their products were replaced with Disneyland posters. All the items in the display area were taken away and the gift shop took over all the area of the old post show area. Finally, the Monsanto "Fountain of Fashion" that dripped oil down strings tightened to it was replaced with a series of sculptures that looked like two plates put together with a snowflake on them. However, the shape of the old fountain/oil lamp was reused in the new sculpture.
Growing up in Los Angeles in the 70s/80s we visited Disneyland on a regular basis and often after school, before "Annual Passes".