What are the big blue pipes seen in German cities like Berlin and Cologne? The ones about a foot in diameter.?

Question

I have seen them down by the Rhine in Cologne and near the Brandenburger Tor in Berlin. But the two locals I asked have no idea what is in them! I never seen them before in a city and am intrigued.

Answer 1

"Lilly" gave you a correct answer...In most German towns, you don't see these pipes, because they're underground - but, you can see them in some other towns.  I live in a small town in Northern Germany and in my town, you don't see them at all.

Greetings from Germany.

Answer 2

You can find these pipes in almost every city in Germany. They transport the thermal energy to supply people/households all over Germany with heat. Actually they were not supposed to be visible in towns, but once they've built these pipes they did not want to move them underneath the earth (for whatever reason).

So actually there should be hot water inside them.