|Primary inflows||From the Hasu River via neighbouring Lake Mikata.|
|Max. length||2 km|
|Max. width||2 km|
|Surface area||4.3 km2|
|Average depth||34 m|
|Surface elevation||0 m|
Lake Suigetsu (水月湖 Suigetsu-ko?, literally "water moon lake") is a lake in Japan, which is one of Mikata-goko (三方五湖?, literally "Mikata five lakes") located in Mihama and Wakasa, Fukui Prefecture (west-central Honshu), close to the coast of Sea of Japan. Since 1993, it has been attracting the attention of scientists because of the undisturbed nature of the water for many thousands of years. It is possible to identify the annual deposits of silt in a similar manner that tree rings are identified.
These varves allow a chronology to be established, because of the leaf fragments embedded in them, which have been used to calibrate the Carbon-14 time scale beyond the range of the absolute tree-ring calibration. In late 2009, the journal Radiocarbon announced agreement on the INTCAL09 standard, which extends a more accurate calibration curve to 50,000 years. The results of research on varves in Lake Suigetsu, Japan announced in 2012 realised this aim. "In most cases the radiocarbon levels deduced from marine and other records have not been too far wrong. However, having a truly terrestrial record gives us better resolution and confidence in radiocarbon dating," said Christopher Bronk Ramsey. "It also allows us to look at the differences between the atmosphere and oceans and study the implications for our understanding of the marine environment as part of the global carbon cycle." Results were published in 2012 in the journal Science increasing the calibration from 12,593 to 52,800 years.  
The only inflow to Lake Suigestu is through a shallow channel from the neighbouring Lake Mikata and there is little outflow. Consequently only the finest sediment comes into the lake. The water is anoxic (deoxygenated) preventing the growth of organisms and due to seasonal variations it is usually but not always possible to distinguish the annual deposits visually. It has taken almost twenty years to overcome the consequent problems, using multiple cores and new detection techniques to complete the sequence.
- Suigetsu 2006 Research Project, retrieved 19 Oct 2012
- Reimer, P.J.; et al. (2009). "IntCal09 and Marine09 Radiocarbon Age Calibration Curves, 0–50,000 Years cal BP" (PDF). Radiocarbon 51 (4): 1111–1150.
- Balter, Michael (15 Jan 2010). "Radiocarbon Daters Tune Up Their Time Machine". ScienceNOW Daily News.
- "Japanese lake record improves radiocarbon dating". AAAS. 18 Oct 2012. Retrieved 18 Oct 2012.
- (2012) A Complete Terrestrial Radiocarbon Record for 11.2 to 52.8 kyr B.P.
- (2012) A New Radiocarbon Yardstick from Japan
- Location, retrieved 19 Oct 2012