A houseboat docked along an island on the Mississippi River in downtown Minneapolis sank Tuesday afternoon, leaving part of the vessel underwater.

The two-level boat was moored along Nicollet Island near the Hennepin Avenue Bridge and under the city's signature Grain Belt Brewery sign when it started "taking on water," said Lisa Kiava, spokeswoman for the Hennepin County Sheriff's Office.

No one was on board at the time, Kiava said.

Part of the red boat is underwater and otherwise surrounded by ice.

The owner was notified and "didn't even know that the boat was in any kind of distress," Sheriff's Office Lt. Art Saunders said.

"Why it's sinking, I don't know," Saunders added.

Authorities have yet to disclose who owns the boat. Letters on its outside indicated that it is named the Naiad.