Carshalton Park has a grotto in it which was once the source of a stream which feeds the river Wandle. The now dry canal bed is obvious in the park, as is the dilapidated grotto.

However, if you look at the bottom of the canal bed by the grotto, you can see 3 very small tunnels. You can enter the middle one as someone has kindly removed one of the bars. A 40 foot crawl takes you to behind the grotto, into a brick-built domed chamber, about 6 foot high, and 4 foot across. It is possible to climb through a hole in the roof, taking you into a larger concrete chamber. A drain lid on the surface leads onto one of the paths in the park (we couldn't lift it). There appears to be a blocked off door in this chamber, if I remeber correctly.

There are other small tunnels leading off from the side of the canal bed, though I would presume these are storm drains.

The grotto: