This may be the shaft that lies between the surface of the ground and the top of the underground tank, a little like the conning tower of a submarine. You would lift the manhole and look down essentially onto the lid of the fuel tank, and in that chamber you might have a build-up of vapour leaking from the tank...

This first page from a German firm but in English:

http://www.zenit.de/envitec/db/query/query-all.phtml?DB=Envi...

For pictures of a 'Domschacht'
http://www.gatw-umwelttechnik.de/images/bilder/bild_dom2.gif

http://www.nulon.de/sites/tanks/sammelgruben/sammelgruben_ma...