Beachcombers in Oahu stumbled across something a little bigger than a seashell or piece of driftwood last week. A large block of concrete washed ashore at Erma’s Beach in East Oahu, puzzling the tourists who found it, and the state and city officials who are investigating it.

The block is just a little smaller than a car, and retained its sharply-angled rectangular shape even after exposure to the elements. Two similar blocks were found on Hawaiian beaches last year.

Officials from the Department of Land and Natural Resources believe that the blocks are most likely tsunami debris from Japan. A propane tank covered in Japanese writing and barnacles washed up at Kualoa Beach Park during tsunami season last year, only a month after a Japanese boat appeared on the shores of Malaekahana Beach.

“It’s common to find debris from other parts of the Pacific washed up along Oahu’s shores,” says Dale Palileo, Online Marketing Manager at E Noa Tours. “This applies to tsunami debris as well, but more concern has been placed more recently on debris from the most recent tsunami due to the potential health hazard brought about by radioactivity. Locals have been asked to report any suspected tsunami debris that they may find to local authorities.”

Local news station KHON2 got enough questions about how a cement block could get all the way from Japan to Hawaii that it included an explanation in their article on the strange discovery.

According to the article, objects float because of buoyancy force, a component of Archimedes’ Principle. Basically, an object that’s immersed partially or completely in water gains buoyancy based on the amount of fluid it displaces. If the mass of an object is less than the mass of the volume of water it displaces, it will float.

This means that if the block is hollow, or made of a lightweight or less dense concrete, it would float as long as it displaced a mass of water larger than its own mass. In fact, the American Society of Civil Engineers holds a competition where University students are asked to create working concrete canoes.

Since Erma’s Beach is owned by the City and County of Honolulu, the city’s Department of Parks and Recreation is working with the DLNR to remove the object, but not before an invasive species inspector examines it for potentially problematic marine organisms that may have hitched a ride. The next step in the object’s removal will depend largely on those findings.

Oahu beachgoers are advised not to touch potentially hazardous tsunami debris and should call or email DLNR to report findings.