Image descriptions - multiple tweets from @A_Daneshzadeh (Dr Arash Daneshzadeh) - content below:
Google is so powerful that it "hides" other search systems from us. We just don't know the existence of most of them. Here's a list of sites you may have never heard of, it's how we overcome the madness:
refseek.com - Academic Resource Search. More than a billion sources: encyclopedia, monographies, magazine
worldcat.org - a search for the content of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need
link.springer.com - access to more than 10 million scientific documents: books, articles, research protocols.
bioline.org.br is a library of scientific bioscience journals published in developing countries.
repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related science.
science.gov is an American state search enging on 2200+ scientific sites. More than 200 million articles are indexed.
pdfdrive.com [no longer working] is the largest website for free download of books in PDF format. Claiming over 225 millions names.