1. A tunnel that has been forgotten, abandoned, or whose location has been lost over time
Archaeologists discovered a lost tunnel beneath the ancient city.
Arqueólogos descobriram um túnel perdido sob a cidade antiga.
2. A tunnel that is no longer accessible or has been sealed off
The lost tunnel connected the castle to the surrounding fortifications.
O túnel perdido conectava o castelo às fortificações vizinhas.
3. In fictional or metaphorical contexts, a passageway that represents a missing or hidden connection
The detective followed the lost tunnel of clues through the case.
O detetive seguiu o túnel perdido de pistas ao longo do caso.