Is this all the year round, Tony, or just at the moment? We could do with more information - what direction does your garden face, is it sunny or in shade for most of the day, what kind of soil do you have, where are you in the UK, etc.

I'd reiterate Spritz's questions above - whether its the same year round is important to know, in particular.

Another question to go with the above is how old is your house? A new build or even up to 30yrs old? Builders had to put the debris somewhere and to remove it cost loads, so they buried it under a couple of inches of top soil in the gardens.

There are two basic categories of possibility. Either rain (or run-off) water is unable to drain away and so sits on the surface, or the water table is at ground-level or is rising up via a spring. In order to answer the question we'd need more detailed info, as outlined above by Spritz.