Rule was stated by Blackburn J in Rylands v Fletcher (1866) LR 1 Exch 265:
We think that the true rule of law is, that the person who for his own purposes brings on his lands and collects and keeps there anything likely to do mischief if it escapes, must keep it in at his peril, and, if he does not do so, is primâ facie answerable for all the damage which is the natural consequence of its escape. He can excuse himself by showing that the escape was owing to the plaintiff’s default; or perhaps that the escape was the consequence of vis major, or the act of God; but as nothing of this sort exists here, it is unnecessary to inquire what excuse would be sufficient. The general rule, as above stated, seems on principle just. The person whose grass or corn is eaten down by the escaping cattle of his neighbour, or whose mine is flooded by the water from his neighbour’s reservoir, or whose cellar is invaded by the filth of his neighbour’s privy, or whose habitation is made unhealthy by the fumes and noisome vapours of his neighbour’s alkali works, is damnified without any fault of his own; and it seems but reasonable and just that the neighbour, who has brought something on his own property which was not naturally there, harmless to others so long as it is confined to his own property, but which he knows to be mischievous if it gets on his neighbour’s, should be obliged to make good the damage which ensues if he does not succeed in confining it to his own property. But for his act in bringing it there no mischief could have accrued, and it seems but just that he should at his peril keep it there so that no mischief may accrue, or answer for the natural and anticipated consequences.
Cambridge Water Co v Eastern Counties Leather Plc  2 AC 264 per Lord Goff of Chieveley at p.309:
Blackburn J.’s statement of the law was limited to things which are brought by the defendant onto his land, and so did not apply to things that were naturally upon the land. Furthermore, it is doubtful whether in the House of Lords in the same case Lord Cairns, to whom we owe the expression "non-natural use" of the land, was intending to expand the concept of natural use beyond that envisaged by Blackburn J. Even so, the law has long since departed from any such simple idea, redolent of a different age; and, at least since the advice of the Privy Council delivered by Lord Moulton in Rickards v. Lothian …, natural use has been extended to embrace the ordinary use of land.
… Rickards v Lothian itself was concerned with a use of a domestic kind, viz. the overflow of water from a basin whose runaway had become blocked. But over the years the concept of natural use, in the sense of ordinary use, has been extended to embrace a wide variety of uses, including not only domestic uses but also recreational uses and even some industrial uses.
It is obvious that the expression "ordinary use of land" in Lord Moulton’s statement of the law is one which is lacking in precision … A particular doubt is introduced by Lord Moulton’s alternative criterion - "or such a use as is proper for the general benefit of the community." If these words are understood to refer to a local community, they can be given some content as intended to refer to such matters as, for example, the provision of services; indeed the same idea can, without too much difficulty, be extended to, for example, the provision of services to industrial premises, as in a business park or an industrial estate. But if the words are extended to embrace the wider interests of the local community or the general benefit of the community at large, it is difficult to see how the exception can be kept within reasonable bounds. … I myself, however, do not feel able to accept that the creation of employment as such, even in a small industrial complex, is sufficient of itself to establish a particular use as constituting a natural or ordinary use of land.
Transco Plc v Stockport MBC  UKHL 61 per Lord Bingham at paras 9-11:
9. The rule in Rylands v Fletcher is a sub-species of nuisance, which is itself a tort based on the interference by one occupier of land with the right in or enjoyment of land by another occupier of land as such. From this simple proposition two consequences at once flow. First, as very clearly decided by the House in Read v J Lyons & Co Ltd … no claim in nuisance or under the rule can arise if the events complained of take place wholly on the land of a single occupier. There must, in other words, be an escape from one tenement to another. Second, the claim cannot include a claim for death or personal injury, since such a claim does not relate to any right in or enjoyment of land. …
10. It has from the beginning been a necessary condition of liability under the rule in Rylands v Fletcher that the thing which the defendant has brought on his land should be "something which ... will naturally do mischief if it escape out of his land" (LR 1 Ex 265, 279 per Blackburn J), "something dangerous ...", "anything likely to do mischief if it escapes", "something ... harmless to others so long as it is confined to his own property, but which he knows to be mischievous if it gets on his neighbour’s" (p 280), "anything which, if it should escape, may cause damage to his neighbour" (LR 3 HL 330, 340, per Lord Cranworth). The practical problem is of course to decide whether in any given case the thing which has escaped satisfies this mischief or danger test, a problem exacerbated by the fact that many things not ordinarily regarded as sources of mischief or danger may none the less be capable of proving to be such if they escape. I do not think this condition can be viewed in complete isolation from the non-natural user condition to which I shall shortly turn, but I think the cases decided by the House give a valuable pointer. … Bearing in mind the historical origin of the rule, and also that its effect is to impose liability in the absence of negligence for an isolated occurrence, I do not think the mischief or danger test should be at all easily satisfied. It must be shown that the defendant has done something which he recognised, or judged by the standards appropriate at the relevant place and time, he ought reasonably to have recognised, as giving rise to an exceptionally high risk of danger or mischief if there should be an escape, however unlikely an escape may have been thought to be.
11. No ingredient of Rylands v Fletcher liability has provoked more discussion than the requirement of Blackburn J (LR 1 Ex 265, 280) that the thing brought on to the defendant’s land should be something "not naturally there", an expression elaborated by Lord Cairns (LR 3 HL 330, 339) when he referred to the putting of land to a "non-natural use" … Read literally, the expressions used by Blackburn J and Lord Cairns might be thought to exclude nothing which has reached the land otherwise than through operation of the laws of nature. But such an interpretation has been fairly described as "redolent of a different age" (Cambridge Water  2 AC 264, 308), and in Read v J Lyons & Co Ltd … and Cambridge Water, at p 308, the House gave its imprimatur to Lord Moulton’s statement, giving the advice of the Privy Council in Rickards v Lothian … "… It must be some special use bringing with it increased danger to others, and must not merely be the ordinary use of land …" I think it clear that ordinary user is a preferable test to natural user, making it clear that the rule in Rylands v Fletcher is engaged only where the defendant’s use is shown to be extraordinary and unusual. This is not a test to be inflexibly applied: a use may be extraordinary and unusual at one time or in one place but not so at another time or in another place (although I would question whether, even in wartime, the manufacture of explosives could ever be regarded as an ordinary user of land, as contemplated … in Read v J Lyons & Co Ltd …) I also doubt whether a test of reasonable user is helpful, since a user may well be quite out of the ordinary but not unreasonable, as was that of Rylands, Rainham Chemical Works or the tannery in Cambridge Water. Again, as it seems to me, the question is whether the defendant has done something which he recognises, or ought to recognise, as being quite out of the ordinary in the place and at the time when he does it. In answering that question, I respectfully think that little help is gained (and unnecessary confusion perhaps caused) by considering whether the use is proper for the general benefit of the community. … An occupier of land who can show that another occupier of land has brought or kept on his land an exceptionally dangerous or mischievous thing in extraordinary or unusual circumstances is in my opinion entitled to recover compensation from that occupier for any damage caused to his property interest by the escape of that thing, subject to defences of Act of God or of a stranger, without the need to prove negligence.
Stannard (t/a Wyvern Tyres) v Gore  EWCA Civ 1248 per Ward LJ at para 48:
Cases of fire damage are likely to be very difficult to bring within the rule because (1) it is the "thing" which had been brought onto the land which must escape, not the fire which was started or increased by the "thing". (2) While fire may be a dangerous thing, the occasions when fire as such is brought onto the land may be limited to cases where the fire has been deliberately or negligently started by the occupier or one for whom he is responsible. Is this not a relic of the ignis suus rule? (3) In any event starting a fire on one’s land may well be an ordinary use of the land.