The Main-Danube Canal (Main Donau Kanal) also known as the Europakanal ("Europe Canal") or the Rhein-Main Donau Kanal (abbreviated RMD) is mostly in Franconia (Upper and Middle Franconia) and partly in Bavaria (Lower Bavaria and two short stretches in Upper Palatinate). It links the Danube with the Main, a tributary of the Rhine, and thus the Black Sea to the North Sea. Its official end to end length is 170.71 km.
This canal was constructed starting in the 1960s and was opened along its full length in 1992. It was originally intended as a freight link, linking – as was said at the time, grandiosely but not inaccurately – the North and Black Seas via the river system of Main, Rhine and Danube. The canal reaches the highest point directly reachable via ocean going vessel in the world. Yet rail and trucking play vastly larger roles in the freight business along its entire stretch and the canal never earned enough in usage fees to pay for its upkeep and maintenance. An unintended but serendipitous consequence of its construction is converting inland cities like Nuremberg into cruise ports, and river cruises have become an important business factor even though the freight business is largely an afterthought. The canal is also a frequently used hiking and biking route with decent trails along its relatively flat banks, with several bridges and underpasses from one side to the other. It is also used by various sports clubs that use rowboats and other human-powered boats. There are also some yacht clubs along the canal.
Interestingly, the canal represents the third completed attempt to link Danube and Main (or their tributaries) for shipping of some kind. The first, Fossa Carolina was built under Charlemagne in the 8th century CE and was long thought to have been abandoned before completion until archaeological findings proved otherwise. The second one Ludwig Donau Main Kanal was built under the rule of Bavarian King Ludwig I who was sceptical of the newfangled railway and instead preferred to have a canal built. While it did carry a few goods all the way to its end in the 1950s, it soon proved too small and too outdated to compete with the railways. While parts of that canal still hold water and the former towpaths where burlaks or horses would tow the ships towards their destinations still exist along much of the way and are today popular hiking and bike paths, the canal is not passable for boats along its entirely length and indeed part of it has been filled in to build the new A73 motorway, including ironically the site of a monument celebrating the completion of the canal.
A secondary purpose of the current canal is to enable the transport of water from the usually rather wet Old Bavaria into Franconia. The Franconian Lake District is a set of largely artificial lakes that also serve this purpose. Construction of the canal was not uncontroversial, in part because of feared environmental damage and because opponents doubted its usefulness as a freight route. As a partial response to those concerns, there have been a few measures of nature conservation along the canal and in its environs and the newer stretches were arguably built with a bit more greenery in mind.
The easiest ways to experience the whole length of the canal is booking a cruise that goes through its entire length or taking a bike trip along the path of the canal. The paths next to the canal mostly serve maintenance needs but have since become a popular bike and hiking path and are even part of the daily commute of some people in "bike-crazy" towns like Erlangen
The Canal links the Main river near Bamberg in Upper Franconia with the Danube near Kelheim in Lower Bavaria. The former is served by numerous regional trains and even the occasional ICE. Nuremberg Airport (IATA: NUE) is the closest international airport to the canal and Nuremberg has a harbor for both cargo and cruises, but it sits more or less in the middle of the canal so starting any trip there only enables you to take half of the itinerary. Kelheim has lost all passenger train service in 1988, but if you head a bit further down the Danube, there are plenty of places that are more accessible.
This itinerary starts in the North at Bamberg even though it can naturally be done in both directions. As mentioned above, Bamberg is far easier to get to than Kelheim. In general, major ports are called Hafen whereas more moderate installations are called Lände.
Given that the canal crosses the European main watershed and passes through comparatively mountainous terrain, there are quite a few locks. The German term "Schleuse" is used here for them. Watching a boat getting lowered or raised can be interesting to observe whether you are on the boat or not. These days most of the locks are unmanned but there are often plaques with information on them that explain the canal and the particular set of locks.
The canal has several points on either side where you can enter or exit with a small kajak, canoe or rowboat and there are a handful of marinas along its length. If you wish to make use of the locks you have to call the operator as they are unmanned. There's usually a clearly marked telephone to call for using the locks.
There are signs on the paths next to the canal that "entering them is on your own risk" and you should rather not fall into the water, as it may be difficult to get out, but other than that you should be safe.
Obviously as this canal links Main and Danube, you can head on along those rivers if you have a watercraft or explore Upper Franconia or Lower Bavaria respectively.