It is often said that Nibbana is unconditioned. But isn't Nibbana to be attained through practice of the Noble Eightfold Path (abandoning desire, meditation, realizing paticcasamuppada etc)? Aren't those practices conditions for Nibbana? What am I missing here :) ?
The practice of the eightfold noble path leads to the experience of nibbāna, just like the act of adverting the mind to the eye door leads to seeing light. Light, the object of seeing, is saṅkhata (conditioned), but nibbāna, the object of supermundane consciousness, is asaṅkhata (unconditioned). So nibbāna isn't the result of the eightfold noble path, the experience of it is.
In the material world alls phenomena arises and passes based on conditionality (as opposed to totally random). In Nibbana there is no arising and passing away of phenomena.
It is true that realising Nibbana is the result of practising the path to realise it, hence if you have realised Nibbana then this is because you practice the path.
When it says that "Anicca vata sankhara" ( Impermanent are all conditioned things). "Uppadavaya dhammino" ( Of the nature to rise and fall). It is saying that things arise dependent on conditions, things "exist" because of the conditions that support them. So we are talking about characteristic nature of things or its attribute. The attributes of nibbana as pointed out above is permanent, does not arise depending on conditions.
In Theravada Buddhism, when we talk about conditions and conditioning there are 24 conditions found in the Patthana
The Patthana Dhamma are packed into 24 Paccaya or 24 conditions. They are:
1.Root condition ( Hetu Paccayo )
2.Object condition ( Arammana Paccayo )
3.Predominance condition ( Adhipati Paccayo )
4.Proximity condition ( Anantara Paccayo )... etc...
Nibbana as stated in the Patthana with my summary below:
square brackets indicate my extra notes
*asterisk indicates I've changed to the English rendering according to Nyanamoli as it appeals to me to be a closer rendering, otherwise it is straight from the Patthana.
Nibbana the term derives from nivana or nirvana. Ni means nikkhanta or liberated from vana or binding. Vana is the dhamma that bind various different lives in the samsara. So nibbana means liberated from binding in the samsara. This binding is tanha. [Thanissaro Bhikkhu uses unbinding in most of his translation instead of using nibbana]
From view point of contemplation [at the time of release], there are three kinds of nibbana. They are sunnata nibbana [*emptiness - attained thru seeing anatta], animitta nibbana [*signless - attained thru seeing anicca], and appanihita nibbana [*desireless - attained thru seeing dukkha]
All 3 are just one same nibbana perceived through different modes of release.
Nibbana is not in one of the 5 aggregates, [it is there all the time but we don't have the mental capacity to see it]. It can be an object of the mind (arammana).
Who can see nibbana? Arahants, and Anagamis only when he enters (*the state of ceasation of consciousness) nirodha samapatti.
[Sotapanas, Sakadagamis and Anagamis only have a glimps of nibbana]
Sankhata dhatu [*conditioned elements] are those whose arising and existence are influenced by one of four causes namely kamma [*action], citta [*consciousness], utu [*climate], and ahara [*nutriment]. Nibbana cannot be influenced by these four causes. Nibbana is asankhata dhatu [*unconditioned element].
I see two ambiguities here which might be the responsible for the confusion.
with the six sense bases as condition, contact;
-- SN 12:1
The above is one link of conditioned arising chain, which explain how many things are conditioned. Would it be correct to, according to the above, conclude that the six senses cause (or produce) contact? That would be awkward.
This quote means that if a "contact experience" is present, then the corresponding sense is present. A more verbose reading of the above would be "six sense bases are a supporting condition for contact".
As another (unrelated to buddhism, and perhaps poorly chosen) example, if you study, you become more knowledgeable. But your knowledge, at any given point, is not constituted by "studies". There is an ambiguity in how we can interpret "conditioned". Conditioned ≠ caused by.
The matter in question is that it is more a statement about a "necessary condition" than a statement about a particular cause.
There is also another ambiguity in play here. Nibbāna is unconditioned in the sense that it has no supporting condition. Echoing yuttadhammo's answer , nibbāna is "one thing" and the experience of attaining nibbāna (which requires all the proverbial effort) is "something else".
I do not have sufficent knowledge to answer the question in a satisfactory way.
So i will instead point you to some great ressources regarding your question.
The first one is a book called "On the Nature of Nibbana" by Ven. Mahasi Sayadaw.
In this book Nibbana is being discussed e.g. "What is Nibbana and what is the meaning of it?, Mental Formation and Nibbana, The Nature and Attributes of Nibbana and many more things regarding Nibbana".
The second source is a great audio dhamma talk called "Nibbana" by Ven. Bhikkhu Bodhi.
Here is also some text material on Nibbana by Ven. Bhikkhu Bodhi. A short quote from the section "Nibbana is an existing reality":
"Regarding the nature of Nibbana, the question is often asked: Does Nibbana signify only extinction of the defilements and liberation from samsara or does it signify some reality existing in itself? Nibbana is not only the destruction of defilements and the end of samsara but a reality transcendent to the entire world of mundane experience, a reality transcendent to all the realms of phenomenal existence.
The Buddha refers to Nibbana as a 'dhamma'. For example, he says "of all dhammas, conditioned or unconditioned, the most excellent dhamma, the supreme dhamma is, Nibbana". 'Dhamma' signifies actual realities, the existing realities as opposed to conceptual things. Dhammas are of two types, conditioned and unconditioned. A conditioned dhamma is an actuality which has come into being through causes or conditions, something which arises through the workings of various conditions. The conditioned dhammas are the five aggregates: material form, feeling, perception, mental formations and consciousness. The conditioned dhammas, do not remain static. They go through a ceaseless process of becoming. They arise, undergo transformation and fall away due to its conditionality.
However, the unconditioned dhamma is not produced by causes and conditions. It has the opposite characteristics from the conditioned: it has no arising, no falling away and it undergoes no transformation. Nevertheless, it is an actuality, and the Buddha refers to Nibbana as an unconditioned Dhamma."
May this be of some help to you.