From AFP:

US to test 700-tonne explosive

The US military plans to detonate a 700 tonne explosive charge in a test called "Divine Strake" that will send a mushroom cloud over Las Vegas, a senior defense official said.

"I don't want to sound glib here but it is the first time in Nevada that you'll see a mushroom cloud over Las Vegas since we stopped testing nuclear weapons," said James Tegnelia, head of the Defense Threat Reduction Agency.

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.