Abstract: A passive detection system is used for monitoring the integrity of abandoned, suspended, and/or decommissioned subsea wells or carbon dioxide sequestration reservoirs, where the system utilises a passive detector that reacts to the presence of a predetermined chemical to release a buoyant beacon. The beacon travels under buoyancy to the sea surface, where it transmits signals to an operator to alert the operator to a potential loss of integrity at the site. The system can include a trigger that initially restrains the beacon, but degrades in response to contact with the predetermined chemical. The system can include gathering and concentrating devices that collect the leaking chemical and direct the collected chemical towards the trigger, facilitating contact between the chemical and trigger and resulting degradation. Once the trigger has degraded sufficiently, it loses structural integrity and fails, releasing the beacon.