Marine Lakes are a rare natural phenomenon which can be found at karstic coasts. Marine lakes are water bodies connected to the “open” sea through “weak” connection(s) like cracks or narrow channels. They are relatively small but still big enough to house unique ecosystems. The Adriatic marine lakes are extremely varied and consist of lakes with permanent or periodic anoxic layers on the bottom, sharp chemoclines, pycnoclines, thermoclines, layers with hyperoxic conditions, layers with superdense bacterial communities; red, green, purple or brown coloured waters; some are very transparent and others extremely turbid etc. The organisms in them compose unusual and very picturesque assemblages. Even though they are relatively shallow (down to 40 m), small and easy to enter, SCUBA diving in them is rare. However, they are of great importance to the marine scientists diving in them.