If ḥametz is buried under less than three tefaḥim (c. 24 cm) of stones, dirt, and the like, it is not considered to have been disposed of, and it must be uncovered and disposed of before Pesaḥ. This is because it is possible for a dog to smell it and dig it up.
However, if the ḥametz is covered by more than three tefaḥim, it is considered to have been disposed of and does not cause one to transgress bal yera’eh and bal yimatzei. Therefore, it need not be dug up and disposed of. Bitul ḥametz is nonetheless required, because it is possible that some of the stones will be moved during Pesaḥ, and the ḥametz will no longer be covered by three tefaḥim, causing the person to violate bal yera’eh and bal yimatzei. Likewise, in a case where ḥametz falls into a pit in one’s yard, if it is the sort of pit one does not generally enter, one may nullify the ḥametz without extracting and disposing of it.
Accordingly, if ḥametz is stuck behind a wall cabinet and impossible to remove without first taking the cabinet apart or emptying it out and moving it, one need not remove the ḥametz. In this case, one may rely upon its bitul (SAH 333:19). If ḥametz is located where it can only be removed with some difficulty, one may pour bleach or soapy water on it until it is no longer fit for consumption, and hence no longer considered food, as it will not even be fit for a dog’s consumption, and it therefore need not be removed and eliminated.
A storage room where one keeps articles not in use or merchandise that he does not intend to use until after Pesaḥ need not be searched for ḥametz. It is sufficient to nullify any ḥametz that might be there. However, if one wishes to fill the storage room during the thirty days prior to Pesaḥ, he must search it first, because the obligation to prepare for Pesaḥ has already taken effect. If one did not search it beforehand, it must be thoroughly searched for ḥametz on the night of the 14th (SA 336:1). If it is difficult to move all of the storeroom’s contents in order to carry out the search, one may sell or rent it to a gentile and thus exempt oneself from the obligation to perform bedikat ḥametz there.
In a case where the ḥametz is covered by less than three tefaḥim, one would be required to remove the ḥametz, as I have written, but only if one is certain that there is ḥametz under this particular covering. Even if there is a danger of snakes and scorpions, he still must take a shovel and remove the covering to expose and destroy the ḥametz. However, if one merely suspects that there is ḥametz under the covering but is not certain, he is not required to check for the ḥametz if there is a danger of snakes and scorpions, and is only required to nullify the ḥametz verbally. If there is no danger, though, he is required to check (SA 433:8, MB ad loc. 35). In any situation where one is required to check, he can exempt himself from checking by selling the item to a gentile. It seems that if the ḥametz is likely to spoil before Pesaḥ to the point that it is unfit for a dog’s consumption, or if it is already being eaten by bugs, then even if there are not three tefaḥim above it, it is not necessary to check it.