What is "deep geological repository"

n. A concept that involves the placement of long-lived radioactive waste, often spent nuclear fuel, in rooms excavated deep within stable, low-permeability bedrock

A deep geological repository is a nuclear waste repository excavated deep within a stable geologic environment (typically below 300 m or 1000 feet). It entails a combination of waste form, waste package, engineered seals and geology that is suited to provide a high level of long-term isolation and containment without future maintenance. The Waste Isolation Pilot Plant, under construction in the USA, is the only one which actually contains nuclear waste. But the facility suffered a chemical explosion which led to a serious radiological accident in 2014.