This is how to deal with the most subtle sorts of defilements hiding out in the heart. Hit them hard with the truth of impermanence, suffering and not-self, because these defilements are the essence of sammuti - all our suppositions about the world. They must be crushed and dispersed from the heart, and then we'll see whether the heart has also been annihilated. No... it is not destroyed. The heart has no cemetery. It is undying by its very nature - even when it still has defilements.
The Lord Buddha called this the complete dissolution of the defilements: "the end of danger, the quenching of the fires of greed, hatred and delusion" by the pouring of the Undying Dhamma (amata-dhamma) elixir. With the defilements gone, only spotless purity remains. And it is here in this fully purified heart that perfect happiness is found. All work finishes here; all projects end here. The Lord Buddha said: "Done is the task, fulfilled is the holy life, there is no further work to do." All suffering ends with this right understanding.
The summit of Dhamma is found in this purified heart. The "Buddham-Dhammam-Sangham saranam gacchami" which we repeat to reverently recollect the Lord Buddha, all gather and converge in this pure nature. Concern with questions about the Lord Buddha's parinibbana (final passing away) in India -- "how many centuries ago is it now?" -- will now end. The true nature of Buddha, Dhamma and Sangha will now be evident in "one's own" heart of purity. This is the heart's priceless treasure, where the true state of things is seen and all questions settled.
Where did the Lord Buddha go when he finally passed away? The bodily aggregate certainly disintegrated, following its nature - bodies being the same everywhere. However, the purified nature, Buddho, was not destroyed or annihilated, for it is not confined by time or position. And it is this which we refer to in "Buddham-Dhammam-Sangham saranam gacchami". When we can experience this for ourselves, we will be able to verify with complete certainty that this nature cannot be annihilated.
The arahant disciples understand this. And wherever they may be they are together with Dhamma; with "Buddha, Dhamma and Sangha", having constant audience with the Lord Buddha. Their minds are unshakable because the fires of desire are quenched with the waters of Dhamma. "The quelling and cessation of all conditioning factors is supreme happiness." (Tesam vupasamo sukho.) These conditioning factors are really samudaya, the source of suffering, so when this is ended all that remains is supreme happiness.
The means and the results, the good and the bad, are within all of us who are aware and care. This nature which knows is uniquely suited to all levels of Dhamma, up to and including the state of purity, and there's nothing apart from this knowledge.
Please try to steadily purify this nature that knows, ridding it of all obstructions and oppressive influences. There will then be no need to make inquiries about Nibbana, for having experienced the purified heart all questions will be finally settled.
(Extracted from To the Last Breath - Dhamma Talks on Living and Dying by Venerable Ajahn Maha Boowa Nanasampanno)