In 1972-73 I remember reading a novel I found in the children's section of a local public library, which was a science fiction tale about drilling deep into the Earth's crust, loosely based on a real American drilling project of the 1950s/1960s known as Project Moho. The real project was an unsuccessful attempt to penetrate to something called the Mohorovic Discontinuity Layer, deep underground.
As far as I remember there were other novels in the series, all of which were set in Outer Space and involved voyages of exploration to different planets in the solar system; but I think the Moho Project was the first story in the series. I recall there being about half a dozen books altogether.
There wasn't much linking the various stories, except that they all featured pretty much the same characters (AFAIK there were three or four regulars, all male and all adults).
Unfortunately I can't recall the name of the author. And apart from a vague recollection that the title of the first book made some reference to the Moho project (and may have referred to it, somewhat inaccurately, as 'Project Mohole'), I can't recall any of the titles.
Project Moho was a real life scientific investigation that began in the USA in 1958. As the first book was based upon it, the novel must have been written after 1958 - but sometime prior to me reading it in 1972/73.
Can anyone suggest either a possible name for the author, or for the title of the book? Thanks for any suggestions.