What maybe the possible meaning of the phrase ‘ Herculean task’ as mentioned in the above passage?

Read the following passage carefully and answer the questions given below them. Certain words/phrases have been printed in bold to help you locate them while answering some of the questions. Mission Indradhanush upped vaccine coverage growth rate from 1% a year to 6.7%, a rare success for health. Aptly named Mission Indradhanush (MI)—it started out with vaccines against seven-life threatening diseases and now covers an expanded spectrum—India’s vaccination booster mission is a rare success for the country when it comes to health. In a country that registered a shockingly low 1% annual increase in immunization coverage, the initiative has pushed up annual coverage growth rates to a 6.7%. MI was launched in December 2014, to drive vaccination against TB, poliomyelitis, hepatitis B, diphtheria, pertussis, tetanus and measles, in 201 districts that had very low immunization coverage. By July 2017, 2.55 crore children under two years of age and 68.7 lakh pregnant women had been covered in 528 districts across the country. Recently, the Centre launched the Intensified Mission Indradhanush (IMI) to reach 90% immunisation in 173 districts by December 2018. As per a report, when MI was launched, states had asked for additional vaccines, but given vaccine procurement is mapped against birth cohort—or births during a particular period—it meant that vaccines procured for children who were not covered were present somewhere in the supply-chain. The electronic Vaccine Intelligence Network (eVIN) was developed to track these unused vaccine. eVIN now provides crucial information, via a smartphone, on vaccine stock and storage temperatures across cold-chain points in 12 states. While MI has been an unparalleled success, the need is to replicate it for routine immunisation (RI) under the Universal Immunisation Programme. That could prove a ‘ Herculean task’ given RI intends to cover 2.7 crore children born each year, nearly 10 crore children between 1-5 years and 3 crore pregnant women. Perhaps, the learnings from MI could be incorporated here.