The secrecy of the Eleusinian Mysteries was enforced by the building itself, and by a law that made talking about it a capital matter.
A hall built for concealment
The Telesterion, the initiation hall at the heart of the sanctuary, was nearly square, packed with rows of internal columns and stepped benches along its walls so that hundreds of initiates could stand inside with no clear central sightline, and no window let the outside in. Rebuilt larger across several centuries, most substantially under Pericles in the fifth century BCE, it was built to contain a rite, not to display one.
What may have been drunk
Initiates fasted, then broke the fast with a barley and mint drink called the kykeon. In 1978 the chemist Albert Hofmann, the classicist Carl Ruck and the ethnobotanist Gordon Wasson proposed that the barley carried ergot, a fungus with compounds related to LSD, which would explain accounts of visions during the rite. The idea remains a hypothesis argued from chemistry and comparison, not from any surviving recipe.
How it ended
The sanctuary functioned for close to two thousand years until Alaric's Visigoths sacked it in 396 CE, not long after imperial edicts had already banned pagan sacrifice. No initiate ever described the central rite in writing that survives, so the silence the building was built to keep has mostly held.