Greenhouse Pests

I appear to have some sort of animal burrowing in my soil in my greenhouse.  The hole is approx. 3 inches in diameter and goes down at an angle of about 45 degrees to a depth of about 5 inches.  There is evidence that it burrows up to 8 feet or so.  I cannot find any droppings.  Any ideas please?.

I had the same thing a couple of years ago. It was voles. I'm afraid I used poison. They got a bit out of control in my daughter's garden and they eat roots. Here is a site that explains and gives different methods for getting rid of them. Bother it didn't work. I'll have to send it separately (if it works).

http://landscaping.about.com/cs/pests/a/vole_control.htm

My garden is undermined with the burrows of short tailed voles. Some of the grass is very bumpy where collapse has happened

This might be helpful http://www.northpennines.org.uk/Lists/DocumentLibrary/Attachments/183//Small_Mammal_ID_guide.pdf 

I would've thought that 3" diameter holes were a little large for short-tailed voles - more like brown rats I'm afraid