How big? From Vegas you'll be able to spot the mushroom cloud, "the first time in Nevada that you'll see a mushroom cloud over Las Vegas since we stopped testing nuclear weapons," said James Tegnelia, director of the Defense Threat Reduction Agency.
The Associated Press reports:
Tegnelia said the test was part of a US effort to develop weapons capable of destroying deeply buried bunkers housing nuclear, chemical or biological weapons.
"We have several very large penetrators we're developing," he told defense reporters.
"We also have -- are you ready for this -- a 700-tonne explosively formed charge that we're going to be putting in a tunnel in Nevada," he said.
"And that represents to us the largest single explosive that we could imagine doing conventionally to solve that problem," he said ...
He said the Russians have been notified of the test, which is scheduled for the first week of June at the Nevada test range.
"We're also making sure that Las Vegas understands," Tegnelia said.