Updated with information about English DVD releases of the series! See end for details.
I've been practising my French... although I'm not that fluent, so Babelfish and Google Translate were used to check I wasn't guessing too much. Information in English on current projects surrounding this classic series is limited, so hopefully this entry will be of wider use.
The Mysterious Cities of Gold (Les Mystérieuses Cités d'Or) was a French/Japanese animated collaboration produced at the start of the 1980s. Its storyline was loosely inspired by The King's Fifth, a 1966 children's novel by Scott O'Dell, the 'fifth' referring to the tax levied by Spanish rulers on precious metals that were mined. MCoG only used this as a base, and at heart is a downright weird mix of historical detail and science fiction — the premise being that ancient South American peoples built technological wonders that were then lost (and this being the 80s, nuclear power features in the subtext.) Set in the 16th century, a time of colonisers and gold-lust fantasies such as El Dorado, the plot is often light on real-world consequence: the three main protagonists, all children, are able to evade or deal with the dangers and exploitative adults they encounter. On the other hand, death and suffering aren't entirely shied away from — plus a lot is suggested in dialogue that couldn't be shown. As family entertainment that doesn't dumb down and has an ongoing plot, it's very well-realised.
To the best of my knowledge it hasn't been repeated on a non-terrestrial channel available to the UK since 1995, but there are regular repeats in France. Indeed, it's popular to the extent that four-disc boxsets of the French release are plentiful on eBay (or as cheap as 8 euros plus shipping from French retailers) and you can get phone covers for older Nokia phones. Even so, it's a good few years since the last revival to produce CDs and these (more-so than the series on DVD in English) are what I was after, because one of the reasons MCoG leaves such a strong impression is its music.
Possibly best known for his work on Inspector Gadget, Shuki Levy [connected with Haim Saban as Saban Records] produced scores for Ghostbusters, MOTU, She-Ra, MASK, Jayce and the Wheeled Warriors, Ulyses 31, etc. as well as music for live action TV such as Buck Rogers and Starsky & Hutch. There was enough of a market in France to support tape and LP releases, some of which can be seen at this collector's site.
Neither the 1982 Polydor/Saban vinyl [813 391-1] nor the 1988 Polygram/Rym CD [197 021-2], pictured left, contain a particularly memorable composition that tends to stick in people's minds. named 'Le secret passage' on an official release and 'Le Feu de Saint-Elme' (St Elmo's fire) by fans, both for moments in the show where it was used. The piece in question features low vibrating bass synth overlaid with sparkly stringed sounds, rising and falling, and was used to convey momentous/uncanny/haunting parts of the narrative — it goes right through you, particularly on speakers with clear bass. Those interested can get a version (reconstructed from episodes by Emmanuel Maniez) from http://www.citesdor.com/musiques/fr_musiques_reco.html
The more recent nostalgia-market Animusik/AK Vidéo CD release [AK-11015] that does have them seems to be based on the same or similar reconstructions, due to loss of the original master tapes. This CD was produced in 1998-1999 and includes the 13 tracks from the French LP + mono tracks reconstructions + a French DVD episode of #18. It went out of print in 2000 and copies occasionally turn up on eBay for rather large sums.
An alternative is modern reorchestrations of the music (plus derived works) by a composer working under the name Boub, real name Yannick Rault. These were released through Loga Rythme in around 2002-2003, in time to each receive a second pressing before that company went bankrupt in 2005. They seem to be out of stock here but copies turn up periodically on the French fixed-price eBay equivalent PriceMinister or you might have some joy with this Swiss company for the second. The first CD has the St Elmo's fire track, and it's a good rendition, with the rest of the compositions paying similar obsessively close attention to detail. I managed to pick up copies of both, and each comes with an illustrated booklet introducing characters and artifacts from the series. The second disc isn't as focused or essential as the first, but is also fun.
As for MCoG itself, there are actually two current projects. Bernard Deyriès & Jean Chalopin, original series creators, have been trying to generate backing for a series leading on from the original cartoon for a few years now, so naturally they're a bit pissed off to be beaten to market by another cartoon adaptation of The King's Fifth called Esteban et Zia à la recherche des 7 cités d'or. The latter had three episodes released on DVD (with regular and collector editions) in December 2006, and is projected to continue into a 26 episode series. A rather nifty site has some info on the story and characters, plus a trailer and clips (under bande-annonce) featuring animation and the voice actors singing. Esteban et Zia is closer to the book and minus most of the elements MCoG added, which presumably is why AK Vidéo are clear to make and distribute it. It's still fantasy rather than history, but fantasy of its own.
And as for MCoG2, a little information is available (translated link follows) here — if and when it goes into production, it'll be a 2D/3D hybrid (much as a recent Oh My Goddess TV series mixes rendering and traditional animation style) put out as a full-length film.
Hopefully both projects will find audiences, which would mean a chance of translations, further interest generated in the original series, and possibly a decent re-release of the soundtrack and/or Boub's re-orchestrations.
Other interesting things I stumbled across whilst writing this (apart from loads of people exploiting the lack of English DVDs with fan-makes ranging from sub-TV to DVD in quality) include a PSP game project (wow!) (I later found it mentioned on LesCitiesDor.com — that place is really comprehensive.)
Miscellaneous notes that might help in tracking down music: the Polygram soundtrack CD has the same number of MCoG tracks as the double vinyl album, sixteen, but has two Inspector Gadget tracks as a bonus. The band that performed the music were called Apollo, rather fittingly.
Sources: thanks to http://www.nostalj.com/ and the two première French fan sites, http://www.citesdor.com/ and http://www.lescitesdor.com/ for the music and new series information. I also got some info from http://www.discogs.com/ and a wealth of leads and pointers from http://groups.yahoo.com/group/mcog-list/
There's also a higher bitrate reconstituted version of St Elmo's Fire by someone called RADiX Fan Projects from 2006, currently available at the MCoG-List group if you join and browse the files.
Oh, and those looking for DVDs with English audio might be in luck sooner rather than later, as Play.com have a 'coming' listing added earlier this year. You can find that here.
Update: confirmed by Tim Skutt (he did the unofficial 8 DVD English set that crops up on eBay sometimes) who has been asked by Fabulous Films in the UK to help with an official set. Check out their site to be notified when work is finished. They also distribute a DVD of the first series of The Raccoons! Wonder whether it has the theme tune as stand-alone audio on it... it's possible, as Kevin Gillis is involved. I also dimly recall there being an episode where Run With Us was played in its entirety, and the song had a (typically cheesy 80s) music video, any of which might be included...
Fabulous has an American division, so US MCoG fans should be getting an English R1 DVD set — their website currently estimates February 2008 as a release date for this.
Update 2: as a limited promo for the Play.com release of the boxed set, a soundtrack album (more of a soundtrack EP, by the running time) was released. It includes St Elmo's Fire.