A little lower down the hill, nearer to Maze Hill, used to be a pond that was part of the conduit system – the location though is obvious on the ground – a flatter area, banked at the bottom carved out of the lower slopes of the Park.
There are other possible signs of the remains of the conduits in the same area – around the eastern flanks of One Tree Hill there is what looks like a slightly raised track going up the hill (which has a couple of manhole covers), toward the top of which it has clearly been flattened out – perhaps there was originally another pond or open reservoir here. It would certainly have been in the right area based on a map found by Subterranean Greenwich – more on them later.
The tunnels themselves are built of brick, and are generally about 4 to 6 feet high and about 2 feet to 4 feet wide and generally run between mere inches to some thirty feet or more below ground level. At the top is a rounded or Gothic arch. At the bottom of the conduit walls are gaps in the brickwork to allow water to feed into the conduit.
The conduit system largely stopped being used when the now redundant reservoir was constructed between 1841 and 1844 close to Great Cross Avenue on the western side of the Park. As a result, many of the conduit heads and other above ground evidence was demolished in the second half of the 19th century.
There is a fascinating resource with an enormous amount of information on many of the things covered in this post and on all things below the surface in SE10 and slightly beyond – Subterranean Greenwich. This covers the conduits in much more detail, including a lot of the underground aspects of them.