Here's what chatgpt says:

That looks very much like the top of a buried inspection chamber / manhole cover base, most likely for an old drain or soakaway. Here’s why: The circular, stepped concrete shape is typical of pre-cast inspection chambers The raised inner dome often supported a removable lid (which may now be missing or buried) The central hole/indent can be where a lifting point or fitting once sat The location in a garden is common for old drainage access points (especially in UK properties)

Most likely possibilities Old drain inspection chamber (most likely) Soakaway access cover Redundant septic/greywater feature (if property is older/rural)

What to do next Carefully clear more soil around the edges to see if there’s a frame or pipe openings Tap it — if it sounds hollow, there’s a chamber underneath Don’t smash or remove it yet — it may still be connected to live drainage

Important caution If it is an inspection chamber: It may still be part of your drainage system Opening it could expose deep voids or sewage gases

Quick test If you pour a small amount of water nearby and it disappears toward that point, it’s likely still active drainage.