Thread: What's this u-shaped tube under the engine and why does it have oil on it?

Underneath the car this morning and I notice a little bit of black oily gunk on the black u-shaped tube underneath the car (see picture) and it looks like it's a bit mucky further up.

What bit of the engine is this/should I investigate further for the leak?

IMG_20140118_112259.jpg

And watch that hose as they love to rub through on the engine.
Cable tie a bit of rubber around it where it rubs.

I think it is the low pressure hose from memory.