Your first link has exactly one source, a Google Docs datasheet which eventually leads to this pdf of a report from the UK. The report only mentions in passing that 75-95% of rapes are not reported (the number 79% is mentioned nowhere in the report, at least that Ctrl-F could find), referencing a report from 2002 which I can't seem to find. I'd appreciate it if you could provide that, but ultimately the chart in your direct link provides the 79% statistic with no data other than an offhanded reference. The first reason given for a lack of reporting ("not naming the event as rape") is also pretty important--it means that this person did not feel they had been victimized. When we lump all of those people together into an "unreported crime" category, we are assuming that those women are being silenced (not to say that some of them aren't), and are compensating by silencing those of them who are being honest. I would question their methods in defining rape and surveying about it.

I'm familiar with RAINN, and their source for reporting info is the National Crime Victimization Survey. On page 5 it shows that in 2011 and 2012 approximately 30% of rapes were reported. (In 2003, the only other year with data provided, 57% were reported, and the report states that a smaller percentage of all violent crimes have been reported over time--I'm curious as to why that is.) I would also like to see the split by gender in terms of reports, since we know that men are far less likely to go to the police about rape, and since women get so much of the attention when the topic comes up.

The third article reads like clickbait (and spouts a lot of anecdotes to appeal to pathos) but links to a report in the UK suggesting that around 15% of rape victims in the UK reported their victimization. The report also warns that the sample sizes for many of these statistics are rather low.

None of these numbers are particularly out of line from report and conviction numbers I've seen for other crimes, and these reports still don't account for (or even mention) false accusations, which varying studies estimate to be anywhere from 1-50% of reports.