What we fail to realize is that the ocean is always changing. What lies under water today may have been dry land during the last ice age. Just look at the seafloor map of the region near these ruins. You can see for yourself how this entire region is elevated, and would have been dry land during the last glacial period, when fresh water was locked up in glaciers and ocean levels dropped word-wide by hundreds of feet. And don't think that this ice age was the only one, nor even the biggest one that the earth has experienced over the course of geologic history.
Ice ages come and go on a regularly basis.
It should come as no surprise then that areas currently covered by 100 to 300 feet of ocean water were probably dry land in the not so distant past, and as such were home to ancient civilizations that predate our earliest historic record. And these valleys are in evidence at least 6 times during the past 800,000 years. The most recent took place about 10,000 years before present while the other took place roughly 140,000 to 120,000 years before present. The valley stretching between the two peaks had some feeble attempts to elevate the global tempratures, but not enough to raise the ocean levels to where they are today. This means that shallow areas of ocean, especially at the edges of the continents would have been dry land for almost 100,000 years.