caves of madagascar
Africa, in general, doesn't have a lot of caves despite having so much surface area. Fortunately, there's Madagascar. Madagascar has a significant karst region in the north with over 100 kilometers of known horizontal passage. There's likely plenty more here and elsewhere in this vast nation.
However, there's little info on the internet. Planning a trip here would be a serious challenge, and I reckon the only place to start would be by contacting the French cavers that have come here. There's have been some cave diving efforts here, I believe by the French as well, who might know more.
Transport in Madagascar is reputed to be horrendous, as is sickness, heat, creepy-crawlies, and bandits. It's a fascinating country, and accessing its caves looks to be quite the challenge. I'm excited for it, someday.