When we bought the property, there seemed to be a lot of manholes around the place that didn’t have proper covers. The covers were either old bits of plywood or large slabs of stone.
The rotten plywood covers (mainly on the septic tank) have been replaced with proper cast iron covers. This left us with one manhole in the top field that covered the access to the soakaway. We have no idea how old this soakway is, but when we tested it (by scientifically throwing about 40 gallons of water down it), it seemed to work. Rather than digging a new one, it made sense to reuse it and the rainwater from three drainpipes on the new extension flow into this soakaway.
This was originally covered by a large slab of stone and was covered by soil. We found it when we were digging the foundations for the extension. We have had to lift the height of the manhole so that the new cover is at ground level. This entailed an extra row of concrete blocks on edge. Then we were ready for the new manhole cover (and frame). This was cemented in and once it has all set, we will back-fill around the edge to bring it up to ground level.