Corrugated

I visited a local drain that I'd noticed a while ago but not visited due to my low expectations of it. I parked above an entry, climbed through a hole in the fence someone had prepared earlier and scampered down the rocks to be faced by rather large parallel tunnels.

Navigating through thick mud, I walked down the left tunnel. It soon came to a large chamber, intended as a catchment for the above car park.
Dead-end

With this being the end of the left tunnel, I tried my luck in the right one. At about the point where the left tunnel ended, this one lost height.
Lower

The tunnel curved sharply to the right and eventually came to a large catchment, presumably for a nearby lake.
Catchment

Satisfied with what I'd stumbled upon, I headed back downstream to see what more was to be found. The downstream section, while not as high, was still comfortable walking. It didn't continue as far though, opening up to a popular restaurant.
Swamped

Although the tunnels seemed like they'd never even come close to full capacity, it was obviously expected to be a possibility, with the lower car park containing a plethora of stormwater inlets.
Inlet