Underground London - Carshalton Park grotto

 The grotto is a small manmade structure in Carshalton Park, in the London borough of Sutton. According to Wikipedia, the grotto was built in 1724. It is located in the southeast corner of the park. A now dry canal extends north from the grotto and can clearly be seen on this Google image -

The canal extends along Carshalton Place, visible on this Maps.Me image -

I walked in from Carshalton High Street and this is the vertical green strip, north of Talbot Rd -

An looking back -

This is the canal section between Talbot Rd and Ruskin Rd, looking south -

There is some water at the southern end, by Ruskin Rd -

This is the main area, looking towards the grotto -

And the grotto -

Access into the grotto is blocked by locked gates. I couldn't see much looking in through the centre gate, apparently the roof was covered with shells when built. 

Looking back down the canal from the top of the grotto, not sure what the concrete cap is in the second photo -

It must have looked quite attractive in the days when water was flowing out of the grotto -

There is a good history of the park and grotto on yourlocalguardian . And a short feature on Wandle Valley . According to Historic England, the grotto is a listed building.