I have been down Riesending twice. It is one of Europe's greatest caves, an utterly magnificent and still growing system with every kind of challenge that is 1,150 metres deep and 21+ km long. I went with German cavers who are excellent company and very competent. It was clear from talking to them there are some other excellent systems in the country, in the Alps, Swabia, the Black Forest and elsewhere. One highlight must be the Blautopf, a huge and richly decorated river cave that used to be accessible only to divers, but to which there is now a dry entrance. I'd love a trip down there! Martyn Farr's Darkness Beckons has some alluring photos of it.