Answer by taking a quote from the following article:

Terence Kemp McKenna (November 16, 1946 - April 3, 2000) was an American ethnobotanist, mystic, psychonaut, lecturer, author, and an advocate for the responsible use of naturally occurring psychedelic plants. He spoke and wrote about a variety of subjects, including psychedelic drugs, plant-based entheogens, shamanism, metaphysics, alchemy, language, philosophy, culture, technology, environmentalism, and the theoretical origins of human consciousness. He was called the "Timothy Leary of the '90s", "one of the leading authorities on the ontological foundations of shamanism", and the "intellectual voice of rave culture". McKenna formulated a concept about the nature of time based on fractal patterns he claimed to have discovered in the I Ching, which he called novelty theory, proposing this predicted the end of time in the year 2012.

In 1965, McKenna enrolled in the University of California, Berkeley and was accepted into the Tussman Experimental College. In 1967, while in college, he discovered and began studying shamanism through the study of Tibetan folk religion. That same year, which he called his "opium and kabbala phase" he traveled to Jerusalem, where he met Kathleen Harrison, who would later become his wife.  In 1969, McKenna traveled to Nepal led by his interest in Tibetan painting and hallucinogenic shamanism. He sought out shaman of the Bon tradition, which predated Tibetan Buddhism, trying to learn more about the shamanic use of visionary plants. During his time there, he also studied the Tibetan language and worked as a hashish smuggler, until "one of his Bombay-to-Aspen shipments fell into the hands of U. S. Customs." He then wandered through southeast Asia viewing ruins, and spent time as a professional butterfly collector in Indonesia.  After the partial completion of his studies, and his mother's death from cancer in 1971, McKenna, his brother Dennis, and three friends traveled to the Colombian Amazon in search of oo-koo-he, a plant preparation containing dimethyltryptamine (DMT).  Instead of oo-koo-he they found fields full of gigantic Psilocybe cubensis mushrooms, which became the new focus of the expedition. In La Chorrera, at the urging of his brother, McKenna was the subject of a psychedelic experiment in which the brothers attempted to bond harmine (harmine is another psychedelic compound they used synergistically with the mushrooms) with their own neural DNA, through the use of a set specific vocal techniques. They hypothesised this would give them access to the collective memory of the human species, and would manifest the alchemists' Philosopher's Stone which they viewed as a "hyperdimensional union of spirit and matter". McKenna claimed the experiment put him in contact with "Logos": an informative, divine voice he believed was universal to visionary religious experience. The voice's reputed revelations and his brother's simultaneous peculiar psychedelic experience prompted him to explore the structure of an early form of the I Ching, which led to his "Novelty Theory". During their stay in the Amazon, McKenna also became romantically involved with his interpreter, Ev.  In 1972, McKenna returned to U.C. Berkeley to finish his studies and in 1975, he graduated with a degree in ecology, shamanism, and conservation of natural resources. In the autumn of 1975, after parting with his girlfriend Ev earlier in the year, McKenna began a relationship with his future wife and the mother of his two children, Kathleen Harrison.  Soon after graduating, McKenna and Dennis published a book inspired by their Amazon experiences, The Invisible Landscape: Mind, Hallucinogens and the I Ching. The brothers' experiences in the Amazon would later be the main focus of McKenna's book True Hallucinations, published in 1993. McKenna also began lecturing locally around Berkeley and started appearing on some underground radio stations.

What is oo-koo-ee?
a plant preparation containing dimethyltryptamine (DMT).