are yet known vs is yet known

Some examples from the web:

• These were wholly unknown in the time of Job, and the discovery of some of those laws - for only a few of them are yet known - was reserved to be the glory of the modern system of astronomy. The suggestion of the great principles of the ...
• The sites selected for NHA designation in this theme cover a significant range of palaeontological interest and importance. No genuine Precambrian fossils are yet known from Ireland, and only a small handful of sites are known with significant ...
• The different types of DD are caused by many genetic defects, not all of which are yet known. Some of the DDs have been given different names based on various symptoms but actually may be caused by defects on the same gene.
• Although no heritable mutations for oncogenes are yet known in man, there are reasons to believe that they could occur. On the other hand, heritable mutations of anti-oncogenes are well known and apparently account for much, or all, ...

Some examples from the web:

• The extent of the damage is not yet known [has not yet been ascertained]. 被害の 程度は未詳である. - 研究社 新和英中辞典. The motive for the murder is not yet known. 殺害の動機は明らかではない。 - Tanaka Corpus. Not much is yet known about ...
• Although non-farm enterprises are ubiquitous in rural Sub-Saharan Africa, little is yet known about them. The motivation for households to operate enterprises, how productive they are, and why they exit the market are neglected questions.
• 2 days ago ... So is yet known as like, the disrespectful player ???? What. Meta Lobster posted.. . I'd imagine that due to shiek's speed and fast animations, they're way way way more sensitive to lag than tons of other characters. Hence the ...
• May 31, 2015 ... Not much is yet known about Nintendo's next console, NX, but it seems Nintendo is putting many of its eggs into its basket. Although the Wii U has been out just two years, the NX looks to be right around the corner, and now ...