This is one of the coolest things on the planet. In the 1800's, German engineers tamed the fickle Isar River. Before, it would meander unpredictably through the centre of the city--not dangerous, but prone to annoying floods and unsightly dry patches. The city engineers sent the flow through two watercourses; a wide, majestic stretch above ground, and another in a narrow channel under the city. The narrow channel would flow faster, and prove useful to power a mill in the centre of town.
The urban surfer's dream: Convenience surfing, right in the centre of town--under a bridge on the historic Prinzregentenstraße, just next to the Haus der Kunst. Many stop for a quick tube on the way to nearby Ludwig Maximillian University.
For added convenience, take advantage of the smoker's bench while you wait for the next wave. Note the faithful mundhundschen. (A free iTunes song to the first reader who gets the pun)Being German the düdes and chicks wait politely for their turn, though some can't resist giving their buddies a playful splash. Of course, he'll be reported to Chancellor Merkel for antisocial attitudes.
A little Munich treat you won't necessarily find in the guidebooks.