What are the elements of a plaintiff's cause of action under the attractive nuisance doctrine under the trap theory?

[51 Cal.2d 105] In Ward v. Oakley Co., 1954, 125 Cal.App.2d 840, 271 P.2d 536, 540, two of the plaintiff's minor children (of an undisclosed age) were drowned when wading in a slough on the defendant's premises. The mud under the surface of the water consisted of a deposit of fine sand and colloidal materials. The complaint alleged that when the children waded into the slough, they were dragged and sucked under the water by the mud; that the dangerous condition of the area was not apparent or obvious to children and that it was practical to fence the slough and to take other precautions. Six children had previously drowned in the slough. It was held that the attractive nuisance doctrine did not apply to bodies of water but that recovery might be had if a 'hidden or concealed trap' could be shown. The opinion states the essential elements of a cause of action under the trap theory to be: '(a) that the dangerous hazard causing death be artificially created by the owner, (b) it be so concealed as to constitute a trap, and (c) that it can be readily guarded without destroying its usefulness or placing upon the owner an undue burden.' The court reviews many cases bearing on this subject and concludes that the peculiar characteristic of the mud was that of a natural rather than an artificial condition.