Official name Wat Saam Prasob วัดสามประสบ "Temple at the confluence of three rivers" (Song Ka Lia, Bi Khli and Ran Ti) or Wat Baan Gao วัดบ้านเก่า "Old house temple", also called Wat Jom Naam วัดจมน้ำ "Temple flooded by water" or Muang Badan เมืองบาดาล "city in under(water)world" (there are more sunken buildings). It is visible from Wooden Bridge.
The temple is half under water in rainy season, but on dry land in other seasons. It is connected to main land, but there is no road. Only access is by boat.
15°06'55.0"N 98°27'41.6"E 15.115264, 98.461567
Pictures taken from the hill that has a village (Wat Baan Gao วัดบ้านเก่า, didn't see any car or mocy):
Clock tower and other building still flooded (there are pictures on the web where these two are on dry land):
There is another sunken temple I know of, in the Gulf of Siam in Samut Prakan province (undertow guides Bangkok off the grid p.103/104 and http://www.bangkok-daytrips.com/a-thai-temple-in-the-sea/ and http://www.bangkok-daytrips.com/bangkok-day-trips-temple-in-the-sea/).
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