Ecstatic moments in Mahler's symphonies usually follow from the gradual build and resolution of a dominant chord. Mahler's imaginative orchestration pushes these arrivals to intense emotional highs. In one of my favorite moments, two piercing high notes of the piccolo push the orchestra over into this resolution and seem to pierce through the very fabric of our consciousness.
I'm talking about the first movement of his 9th symphony where we are first lulled with quiet and gorgeous repetitions of a sigh figure (one note falling a step down to another). A darker, more turbulent theme in minor eventually propels to a fanfare and this dominant chord. As the orchestra reaches the threshold of its loudest possible dynamic, we hear the piccolo way up in the stratosphere, shrilling pushing us over the top with one note, the note of the sigh motive. The harmony then resolves, but the note in that register does not resolve! (It is resolved an octave lower.) So we are still suspended. But the really great moment happens just a second later when the sigh figure repeats and the piccolo plays yet an even higher piercing note (a high A—remember those A's that began Mahler's 1st symphony?). That note too is left resonating in our ear as the orchestra resolves it again an octave lower.
What an imaginative wrinkle to the most basic harmonic progression in music (dominant-tonic). The harmony resolves, the melody does not. This becomes a controlling musical idea for the entire first movement. Only when it ends, do we finally hear the melody resolve in the highest register of the orchestra.
Here is the excerpt from those piccolo notes with the score below (piccolo notes highlighted in red).