Because the grass wouldn't grow very well in a part of my back lawn (and area which looked like a rather ugly 'hump'), I decided to excavate. Having found concrete about 10cm under the turf I eventually found a concealed manhole cover.

On investigation I seem to have found a disused cesspit made of concrete, with iron supports inside, measuring about 5 metres long, 3 metres wide and 3+ metres deep. It seems to be totally empty albeit there is 30cm or so of water at the bottom. I do know that my house was connected to mains sewage about 15 years ago, and I had assumed the cess tank would have been filled in or removed. I can not see any outlets, which has led me to conclude this is not a septic tank. There is one inlet pipe which is not connected to anything as far as I can tell, I suspect the water seeps in through this pipe from the garden above.

Does anyone have any advice as to what to do with this, and who can do it without spending £000s? It looks horribly dangerous as well as being an eyesore over which grass will not grow properly.

The options seem to be:

1. Fill it in, which would take many, many tons of rubble, and would not solve the eyesore/grass problem;
2. Remove it, which would probably be very expensive and would still leave an enormous hole needing to be filled in anyway;
3. Knock it in on itself and then bring it up to ground level with more rubble as needed, then cover the final 30cm or so in ballast/topsoil. Trouble is, that looks incredibly dangerous and difficult to achieve.

Thanks in advance for any advice