A two-level houseboat on the Mississippi River was found sinking Tuesday, the Star Tribune reports.

The Hennepin County Sheriff's Office says it's unclear why the 40-foot boat moored along Nicollet Island began taking on water.

Rushika and Christopher Hague are the owners of the "Naiad." KSTP reports the couple has authored two books about the history of Nicollet Island.

When sheriff's deputies notified the Hague's, they had no idea their boat was under any distress. No one was onboard when the vessel began to sink.

Nearby residents say the red houseboat sitting beneath the Grain Belt sign has become somewhat of a landmark for thousands of drivers who pass over the Hennepin Avenue Bridge everyday.

The couple is arranging to have the boat removed from the water.