A massive ransomware campaign sent out millions of phishing emails earlier this week, and cybersecurity experts are calling it one of the biggest malware operations of the year. Locky, which surfaced in February 2016, demands that users pay ransoms via Bitcoin, which can hide transactions from authorities. Usually, the ransom demand is from $400 to $800. One Bitcoin equals $4734.48.More than 23 million messages infected with Locky were sent in a 24-hour period on Monday. The attack was identified by online security firm AppRiver, which called it "one of the largest malware campaigns seen in the latter half of 2017."The emails contain various subject lines, including 'Scans,' 'Please Print' and 'Documents.' All contain ZIP files with Locky hidden inside. If recipients click on them, the ransomware downloads to their computers, encrypting and freezing all files. Then, they're asked to pay for a special type of software that restores the locked files. Locky has the potential to pull in big money for hackers, even if only a few people fall victim to the emails. Unfortunately, cybersecurity experts haven't developed a way to unlock impacted files without having to pay ransom.Locky is one of the most prominent types of ransomware and was one of the most common varieties of malware used by cybercriminals last year.