Upon his enlightenment, Buddha touched the ground.* He asserted that “Earth is my witness,” regarding the fact of his enlightenment. There are layers of meaning in this gesture which are not usually expanded upon in the Dharma. Generally, the gesture is taken at face value as the response by the Buddha to Mara’s attempt at derision. If Buddha had taken Mara’s ego-centered bait, it would have shown him not to be supremely enlightened due to taints of ego still present within him. But no ego remained. The ground shook with his purity as he touched it.

Layers of meaning:

• Pure, unwordable, unfathomable, constant, enlightened awareness is the ground state of all beings and all existence. We simply have not asserted this to ourselves such that we, like he, have awakened to what already IS. The ground shook because, as the Buddha called it as witness to his buddha nature, he was himself the witness to the very substance of ground and “all that appears and exists” as self-same buddha nature. This is not a statement of “I am buddha nature and existence is buddha nature too.” That would be a statement of duality. Instead, the ground shook because the vibration of its self-same nature being buddha nature was known and known as not different in any way from the only awareness of buddha nature.
• The ground is one’s practice. One’s practice can be any practice at first, and maybe so for a long time. Through most of the path of spiritual maturation, what is important is cultivating the habits of the Path such as diligence, selflessness, kindness, self-reflection, and following a path of wisdom, not just being a follower. But, at some point when these qualities are themselves a foundation, a set of habits of the self, then a meditation practice of mindfulness-insight can be cultivated. Insight was the ground of Siddharta’s meditative analysis of suffering, its nature, its cause, and whether or not the cause could be ceased. Therefore, when the Buddha touched the ground, he was stating that his enlightenment came as a result of the foundation (ground) of certain causes and considerations that he engaged and furthered. Meditation was the method, and mindfulness – which he perfected – was the technique. Meditation, as practiced by ascetics of various schools of yoga and meditation, did not divulge the causes of suffering as Siddhartha meditated for six years, but mindfulness-leading to-insight did. Therefore, it is the ground of true inquiry into the Path overall, into oneself, and is the only practice that will win the great awareness.
• The ground is our ground state. It is our current starting point. No matter where we are,  the situation, circumstance, or description of the moment – it and it alone is the perfect soil to be cultivated (by mindfulness) because it already is the ground of pure awareness. In other words, and just as in nature, the ground is the ground, whether in a low canyon or at the top of a mountain. The ground is the ground. This is a reference to the spiritual Path or the Path of spiritual maturation. Regardless where we are along the Path, it is where we are: we are not further ahead – despite desiring so or fabricating through ego that this is so, and the same is true for thinking oneself immature on the Path. Ultimately, none of that matters, but does matter functionally concerning the practices and teachings that will best suit one’s capacity, desire, and motivation. Nonetheless, the ground is the ground. This moment and how we “stand” in this moment (in delusion or choosing some awareness), and acting, speaking, responding, etc. from that ground is the Path while also being the practice of the Path. Buddha knew this and was calling attention to it.
• Building upon that, then, “All that appears, all that exists, all of samsara and nirvana has one ground, two paths, and two results.”** No longer were fabrications in the Buddha’s mind, nor delusions about this or that, or reality. Therefore the ground of awareness, for the Enlightened One, was/is the only ground. For us ordinary beings, all that appears and all that seems to exist as identifiable and recognizable was no longer perceived as separate in identity from his mind-stream. To the Buddha, all that appeared in his daily awareness, and all that had the contrivance of existing as reality was/is simply his state of Awareness-Being. It all is his MIND. (It’s hard to give words to or make sense of.) But the point, for this list of “grounds” is that mindfulness and resultant insight used and lived to fullness leads to the only result possible: Ground-Awareness: awakened awareness as the only ground of Being, thus Awareness-Being as the only state of Being. “There is one ground.”
• Thus “existence is witness.” The ground represents all existence. One’s existence as well as how one exists within existence is witness to the ground awareness that one is living, or collective humanity is living. Existence is, then, the glaring evidence of when ground of awareness is a functional delusion, ie. the illusion of duality expressed through separateness as the ground of one’s reality and awareness.

Therefore, the Buddha’s touching of the ground upon enlightenment is a highly significant act. It is a call and a reminder, a witness and injunction. It points to the Path, the method and technique that will accomplish the Path (with both its personal and universal beneficent results), that every moment is the Path, and that Awareness-Being is already the ground of Awareness and Being. Simply, be the ground.

* This is called the bhumisparsha mudra in Sanskrit.

** This is the first line in The Aspiration of Samantabhadra.