Early on, Google had an advanced search feature easily available from any search results page, allowing one to easily refine their search.
Since then, they've incorporated a lot of inline shortcuts to allow one to perform much of the same funtionality, though much is still missing.
At some point, the advanced search options button was moved inside the settings menu with the gear icon. After several years of believing Google had just completely gotten rid of the advanced search page, I found it had been hiding in this menu.
This extension re adds the Advanced Search button to the toolbar to make it more easily accessible.