"Then we don't blow it up." Izzy decided, still studying the hologram. "The laws of physics still work here, mostly. If we make a big enough boom underneath it then it doesn't matter how deep the shield goes, the Spire won't have anything supporting its foundation. Without something to rest on it'll collapse."
It was a crude solution and she didn't like it, but she didn't see much way around it. They probably wouldn't be able to protect the surrounding buildings, which meant making sure everyone was evacuated when they set the thing off.
"We're only going to have one shot at this." Left unsaid was that they'd probably all be dead if it didn't work.