8. In my opinion it should not be held that either respondent had knowledge or means of knowledge of the existence of the nuisance on 30th May 1968. Clearly, on the assumption that I have made as to the source of the oil, both respondents had means of knowledge, if not actual knowledge, of the existence of the pile of rubbish, but neither had actual knowledge of the existence of a receptacle of oil. The question then is whether an occupier of the land, if exercising due diligence, ought to have known of the existence of a potential nuisance. The obvious presence of the pile of rubbish was not in itself sufficient to provide means of knowledge that there existed a potential danger to users of the highway. A heap of rubbish may constitute an actual or potential nuisance in various ways - it may be likely to collapse on to a passer-by; it may harbour vermin; it may contain filth. None of these considerations was relevant in the present case. The potential nuisance lay in the possibility that the pile of rubbish would in some way cause oil to flow from a container in or under it. However there is no evidence that such a possibility could reasonably have been foreseen or that a reasonable inspection of the premises would have revealed the existence of any such container. It seems to me impossible to decide on the evidence whether the receptacle which I assume to have existed was in the pile or under it. The quantity of the oil which emerged suggests that the receptacle must have been under the rubbish, since it is unlikely that a pile of the size in question would have held a receptacle large enough to provide as much oil as had flowed and continued to flow from it when Inspector Bourke saw it. On the other hand, it is difficult to imagine what would have caused the oil to flow in such quantity from an underground receptacle. Any conclusion one way or another would in my opinion be no more than conjectural. If the receptacle was underground, it might well have been necessary to move the entire pile of rubbish in order to discover it. It seems to me that the duty of an occupier to take reasonable steps to prevent the continuance of a nuisance created by someone else does not require him to remove every heap of rubble on ground where demolition is taking place, on the off chance that it may hide a potential nuisance when there has been nothing to suggest that it does so. The fact that the land had formerly been used as a service station was not enough to give the occupier presumed knowledge that the pile of rubbish might cover a receptacle filled with motor oil. There was nothing about the nature of the pile of rubbish, or its history, that would have indicated to a reasonable man that it was likely to cause any public nuisance. For all these reasons in my opinion it was not proved that either respondent knew, or should have known, that there existed on the land a nuisance or potential nuisance - that is, that there was a reservoir of oil in or under the pile of rubbish which, in certain circumstances, might overflow on to the footpath causing danger to persons passing along it. (at p555)