Timewheel

Budapest's Timewheel (Időkerék in Hungarian) could be the world's largest hourglass, taking an entire year to drain completely.

Literally, a giant wheel designed to mark time, the public installation was built in 2004 to commemorate Hungary's inclusion into the European Union. The giant wheel is slightly concave, sloping towards the center choke point that marks the upper and lower grain reservoirs. In the case of the Timewheel, the particles that fall to mark the passage of time are tiny pieces of glass which trickle through the clock with the help of a computerized system that keeps the timing perfect. Both the upper and lower reservoirs can be viewed through large triangular panes of glass in the wheel. 

Every December 31, a team of four people use thick steel cables to rotate the Timewheel 180 degrees on its built-in rails, resetting the chronological monstrosity for another year. Even with multiple helpers, the process takes up to 45 minutes to complete the half-rotation.     

The installation is located in a public park and visitors can come by and try to spin the titanic granite wheel for themselves. 

Update December 2018: There is currently construction in the park. The timewheel is within the fenced-off area obstructing the view.

Update September 2022: The Timewheel no longer exists.