This is what the hibernaculum I built has become known as, it is preferable to 'The Grave' as my husband kept calling it to begin with.
A hibernaculum is a place for mostly reptiles to come and hibernate in over winter. Hedgehogs could also make use of it, and maybe even small mammals would during the summer.
Mine consists of several small chambers containing garden stones and organic matter that will rot down over time. I will probably never know for sure if it gets used or not as I'd have to rip it open to find out.
All my materials are recycled and didn't cost me a penny :o)
Thanks to Margaret at work I gained 10 large granite rocks that had been randomly scattered about the garden of the house she had just moved into. These were placed where I had killed the grass off on a rather redundant section of lawn
From a skip at work I got two sizes of plastic tube and used these to make entrances for different sized creatures.
A selection of wood and bark placed inside had been saved from from the dead Leylandii hedge that I took out last year.
The old paving slabs came from my parents, who were please to get rid. I had found the roofing tiles buried below the old hedge (for some inconceivable reason) and the old bricks were from my garden and my parents'.
The roof was a rather careful balancing act as I wanted it to be strong enough and not collapse should the hibernaculum have to be stood on for any reason - although this is strongly discouraged. I did test the strength out personally though and it was sound at the time of building.
As I removed the turf from the area that was to become my pond I layered it over the hibernaculum and carefully covered all the gaps so that loose soil wouldn't pour in.
Lastly the mound was covered in soil from the beginning of my excavation. I planted a wildflower seed mat on the top, added some red clover and creeping thyme seeds to halt the slump of soil on the sides, and also some cowslip plug plants where I had run out of wildflower mat.
Water thoroughly and wait...