A new and potentially dangerous twist on the old juvenile pastime of blowing up mailboxes has been invented and the Davis County sheriff's office is concerned about the possibility of casualties, even fatalities, said Capt. Bud Cox.

Juveniles for years have been using a variety of explosive devices, from M-80 firecrackers to powder extracted from shotgun shells, to blow up rural mailboxes.But the newest method, using dry ice and an empty two-liter plastic bottle, is increasing the danger several times over, both to pranksters and victims, Cox said.

Several such incidents have been reported in the past few weeks, which prompted the sheriff's department laboratory technicians to conduct some tests to determine the power the homemade bombs can generate, Cox said.

Containers with dry ice and various amounts of water were tested. As the dry ice evaporates, it gives off carbon dioxide gas, which builds up until the bottle explodes.

The test results, Cox said, were "phenomenal." The detonation of the devices ranged from 32 seconds to over three minutes. The pressure, measured from one foot away, was measured as high as 9.7 psi (pounds per square inch).

"An M-80 firecracker was tested on the same day and generated an overpressure (explosive force) of between 1.3 and 2.4 psi. The two-liter bottle devices are equivalent to approximately one-eighth to a one-quarter pound of TNT," Cox said.

Cox said he initially was reluctant to release information on the new devices, called improvised explosive devices, because he doesn't want other juveniles to hear about it and try it.

But the danger to unwitting victims who may not recognize the device for what it is outweighs that, Cox said.

The wide variation in times it takes for the bottle to explode make it a danger both for those making the "bombs" and for people who may find one in a mailbox before the device detonates, Cox said.

In one incident, Cox said the top of a mailbox blown up by one of the devices was found 39 yards away, demonstrating the potentially lethal force it can generate.

"If anyone should find such a device in his mailbox he should quickly leave the immediate area and call a local law enforcement agency," Cox said.

Cox suggested that two-liter bottles be punctured before being thrown away and parents should caution children about dry ice. Dry ice can also cause burns.

And, the department is encouraging merchants who sell dry ice to be cautious about its sale to juveniles.