The use of X-ray based attacks in earlier generation BMD systems had led to work to counter these attacks. In the US, these were carried out by placing a warhead (or parts of it) in a cavern connected by a long tunnel to a second cavern where an active warhead was placed. Before firing, the entire site was pumped into a vacuum. When the active warhead fired, the X-rays traveled down the tunnel to hit the target warhead. To protect the target from the blast itself, huge metal doors slammed shut in the tunnel in the short time between the X-rays arriving and the blast wave behind it. Such tests had been carried out continuously since the 1970s.