[schoen]

At the risk of spoiling the joke, just curious about how powerful the drones will have to be...

https://www.tarpsplus.com/blue-poly-tarp-sold-individually.h...

"3.5 oz. Per Square Yrd."

  You have: 3 mi 4 mi 3.5 oz/yd^2
  You want: kg
   * 3688250.3

So each drone should be able to lift about a gigagram.

(Also, this plastic doesn't have enough tensile strength to support its own weight when lifted from the corners.)

[HillaryBriss]

Good point! The plan needs refinement.

I guess they could break the job down: use one team of 4 drones to lift multiple 1 square yard tarps.

Let's see, 12 square miles is roughly 37 million square yards. Multiplying by four they would only need about 150 million drones. Quite affordable for today's well-funded terrorist with an Amazon Prime account (as long as they choose 3-5 business day shipping instead of overnight).