Those things holding up the booms that run across the Waikato River in front of the Taupō Control Gates bridge which stop boats going over the control gates.

They look like big plastic floats. They float like big plastic floats. But they're not big plastic floats.

They're actually steel World War II naval sea mines.

Without the explosives, obviously.

Wikipedia says "a naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered