This animated children's series was allegedly based on Jules Verne's novel, but seems to be based more directly on the 1959 film version. Gertrude the Duck, who wasn't in the novel but provided some comic relief in the movie, shows up here as one of the regular cast of characters. This series featured extremely low-budget animation, but had lots of action and some scripts that were hugely imaginative despite their basic implausibility: it seemed as if every week's episode featured yet another lost race of semi-humanoids living in the earth's core, to say nothing of prehistoric monsters.
I saw several episodes of this series at a screening in London in 1972, when someone from Filmation Associates tried to sell the UK syndication rights. Instead of an 'origin' episode to set up the characters, each episode began with a flashback sequence to explain the show's premise. Basically, Professor Lindenbrook wants to explore a subterranean tunnel discovered by the late Arne Saknussem. For some contrived reason, the latter's evil descendant Count Saknussem -- 'last descendant of the once-noble Saknussem family' -- abetted by 'his brute-like servant Torg', doesn't like this. (Torg looks like a troglodyte, and has a permanent grimace ... probably because his name is 'grot' spelt backward.) The count and Torg dynamite the tunnel, trapping themselves inside along with Lindenbrook and his companions.
Lindenbrook's companions are his attractive niece Cindy and handsome young Alec (no mushy stuff, though) as well as muscular Lars (who gets lots of 'by-yiminee' dialogue) and the aforementioned Gertrude the Duck. Since they can't get out the way they came in, they decide to travel DEEPER into the Earth, in hope of finding another way out. All they've got are titchy little knapsacks; nobody ever explains how these people survive with one set of clothing apiece, and no discernible provisions. (Maybe Gertrude keeps laying eggs.) And, unlike coal miners, they never seem to have any problems with the air, the temperature or the humidity.
So, every week they run afoul of underground pterodactyls, or whatever. The four goodie characters (plus the duck) are travelling separately from the two baddies, but somehow Count Saknussem and Torg keep showing up whenever it's convenient for the script. I'll give the writers some credit for just occasionally bringing a bit of genuine science into the scripts. I recall that, in one episode, Professor Lindenbrook deduced that they must be in a tunnel under the Middle East because it was full of petroleum deposits.
Long on action and science fiction, and short on plausibility, this series was mostly enjoyable. Every episode ended with the adventurers finding some clue that they were still on Arne Saknussem's original trail, and would get back to the surface eventually if they just kept on as they'd begun. Unfortunately, there was occasionally some rather stupid comic relief. One episode ended with the adventurers in a cavern, discovering a carved diagram showing Saknussem's route to the centre of the earth. This was Gertrude's cue to squat on the diagram, prompting Lars to note (in his El Brendel accent) that Gertrude was going to the centre of the Earth 'the easy way!'. Apparently this was meant to be funny, because the other characters laughed. I just wish that ONE episode had acknowledged that the characters had difficulty finding a reliable food source down there in the magma. To say nothing of hygiene problems...