Terrified sailors dived for cover in Hamburg, Germany, when what looked like a full-scale U-Boat surfaced on the city's Alster River.
Pranksters caused chaos on the river's Outer Lake with the life-sized replica of a sub conning tower - attached to a pedalo.
Local river police were scrambled after astonished river users dialled 999 to report the lifelike sub, created by pals Marc Aberle 46, Reinhard Crasemann, and Sven-Ole Kramer, both 45.
"It started off as something we might make from a few beer crates but in the end we wanted a complete replica," explained Marc after the maiden voyage of the 'U-Pooly'.
"We didn't think it would cause such a scare, but the police were fine about it - although they said we should have warned them in advance." he added.
The pals took a year working with a local ship-building firm to create the sub and make sure it would not sink on its maiden voyage.