Simplify the following text for a Grade 13 native English-speaking person.
Original: It has been alleged that a second , deeper metro system codenamed `` D-6 '' designed for emergency evacuation of key city personnel in case of nuclear attack during the Cold War exists under military jurisdiction .
Simplified: Although this has not been officially said , many independent studies say that there is a second , deeper metro system designed for emergency evacuation of important city personnel in case of attack .