The West Mebon of Angkor Wat
The Western Baray is the largest of the man-made lakes at Angkor. Although it's not known exactly when the lake was built, the temple in its center called the West Mebon appears to date from the 11th century.
The Mebon temple is reached by boat from the south dike of the Baray. Depending on the water level, you can walk around the Mebon to see the fallen stones.