Well, it depends on what you mean when you say "under". It is true that all the bodies of salt water referred to as oceans and seas are globally linked together. It would be correct to refer to them as all having a common salinity, maybe a little variation, depending on the location of salts and minerals. But, if you are asking if there is a large ocean underneath the crust of the Earth, serving as a large fountain, the answer is no. The oceans are on top of the crust, the closest thing we would have to a huge oceanic reservoir beneath the Earth's crust would be the continual influx of volcanic activity. Were it not for the volcanic activity going on beneath the ocean's surface, the salinity and mineral content dissolved in ocean water would be significantly less. Millions of years ago, all the continents were together in one giant super-continent called Pangaea, leaving all the oceans to be combined into one giant super-ocean, called Pantahlassa.