I'll preface this with the truth and hope it doesn't disqualify the question but rather give context: after reading Journey to the Center of the Earth by Jules Verne, I am amazed at the prospect of venturing into volcanic conduits to explore the interior of the Earth. How deep do lava conduits go, why don't they go deeper, and what kinds of formations or environments exist at their terminus?
I am not new to Earth science but am pretty new to in-depth geology (pun intended) and volcanology. From what I can tell, the type of lava cave which goes the deepest is referred to as a lava tube. Apparently these tubes can be fairly long (e.g. a tube from Mauna Loa goes 50km before terminating[?] at the ocean!) but not very deep (Wikipedia on lava tubes lists a mere 15m!) Why do these tubes not go far deeper?
The deepest conduit from surface to inner Earth is apparently the Kola Superdeep Borehole which terminates at ~12km deep upon reaching higher-than-expected temperatures. I think of magma - liquefied hot rock erupting out of volcanoes - as coming from even deeper than that depth. Therefore, why doesn't lava tubes or other volcanic conduit continue to such deep depths? Do they, but it is too difficult to explore? Do they during eruptions, but soon after cool and fill with solidified rock or eventually collapse and thereby fill with rock?
As noted I'm new but interested to the subject, so any info to help me understand the processes and environment involved in this interface between inner Earth and the surface would be much appreciated.