It all started back in the mid 70's in Goa when ambient was the spiritual enhancers for people living in that region of the world. Eastern Asia generally and precisely a country like India (and the Tibet) are known for their spiritual and eternal effort for maintaining 'the self', discovering it, and the religious beliefs that derives from the roots of human nature and internal truth for being needed a stimulater, an enhancer, and an element of spiritual connectivity with nature and one's self.
Ambient (before being Trance) wasn't very different from what we listen to today, however, it's approved upon that this sub-genre of trance (though wasn't called Trance at that time) is the eldest among the other sub-genres, its discovery though (as well as its popularization) was consequent to Trance's birth.
Many people argue that this sub-genre is not even trance, and to a far extent they might be right with full regard for Ambient not being even 'dance music' in some critics' perspectives towards the old Ambient sound. The awarness that music generally (and trance in our case here) is ever evolving, defended this theory that rose in the 1980's and argued that Ambient is the bedrock for Psychedelic Trance that was brought to life by the Israelis in the 1980's after India started granting visas for those. After visiting the beaches of Goa, they came back to Israel and introduced Psytrance to the world. That is why India is said to be the 'spiritual home' of Psychedelic Trance (thanks to Ambient Trance). The Israelis took Ambient and extracted Psychedelic Trance from it since Ambient is of a flexible musical nature. Enough from Psychedelic connections, back to Ambient.
The introduction of Ambient to the world had recieved a warm welcome with Ambient Trance producers spread across the globe (notably in Israel and UK). It touched alot of musicians thanks to its universal message 'OF humanity and TO humanity' and decided to adopt it composing tunes from their ownselves addressing the most crucial issues in our daily life:'us'. So, what makes Ambient that special and what is it composed of?
Spiritually, an Ambient Trance tune is literally a journey. A journey into an unknown place of your own choice: no limitations, no borders, no restrictions at all. You might want to smile, or think of something, or somebody, maybe dive in memories, or find answers for questions that seemed out of proportion or your own mental capabilities. It simply is a 'massage' for your brain and thoughts, a deeply influencing sub-genre that you can't help but admire and love for what it have done for you. Ironically enough, you might not even notice its impact straight away, but the sense of relieve after this journey is incredible and really undescribtive. Sometimes, beats are added to the natural galactic sounds in Ambient delivering it as 'Trance' but still in a very unique and superb form. Ambient tune elements are skillfully laidout to slowly absorb into you, to decode your emotions and tensions, so it has various sampled sounds from nature and ancient sounds. The tune might also have vocals... slowly chanted, soft voices (in some tunes: children voices) with ever consequent galactic electronic twists; making any slight mistake prone to immediate detection if out of tempo.
Leading Ambient Trance DJs/Producers are Shpongle, Tripswitch, Arcana, Liquid Soul, Water Juice, Triptych, and Hesius Dome to name a few...
To wrap things up, I have only my recommendations for those who never tried this sub-genre. You may not welcome it instantly, but once you do, you're addicted and strongly bonded to it. Every tune is a conversation between you and yourself, a dialogue where you have absolute privilage to put the words of your own.