Send them tear gas. Pump it into every basement in the city.
Cheaper solution: Pump in seawater.
Indeed. How to find the entrances and tunnel networks?
I've always thought pumping city sewage into the air-ducts and entrances would be a good, low-cost strategy. The goal not being to flood them, which would be a huge enterprise, but to make them highly unhealthy and unlivable, even short periods
“Send them tear gas. Pump it into every basement in the city.”
Tear gas?
You are waaay too nice.
I was thinking about gasoline.