The phrase "mammoth effort" means putting in a very big and strenuous amount of work and effort. Just like a mammoth is a huge and powerful animal, this phrase describes a task that requires a lot of energy and dedication. Full definition
These were relatively early days of the digital revolution, preceded by mammoth efforts to bring together the data that would facilitate the phases to come, the launch of some spectacularly successful services and the strategic acquisitions that were conducted. (slaw.ca)
It was no surprise that once the mammoth effort of building a company set in, he became the poster child for burnout — depressed, irritable, uninspired, and unproductive. (inc.com)
Buying individual stocks is risky and maintaining a portfolio of dividend stocks is a mammoth effort. (thefrugalfarmer.net)