Coral fronds wave gently in the ocean currents, occasionally more agitated by the passing of a larger swell nearing the shoreline. Brightly coloured fish, swimming in vast schools, dart among the coral heads, while their larger counterparts swim sedately along the drop-offs, silhouetted against the deep blue of the ocean depths. Eels lurk in the caves, making occasional forays out into clear water on the never-ending quest for prey. The reefs are encrusted with star fish and sea urchins, and the long tentacles of crayfish protrude from beneath the rocky overhangs. In the upper sunlit zones, ammonites jet their way through the water, their large, squid-like eyes taking in the details of their watery realm, their tentacles trailing behind them as they propel themselves through the warm, blue seas. Sharks proliferate, circling in schools near the surface or hunting in solitary stealth in the deep blue vastness of the oceans. A plesiosaur swims by, momentarily blocking out the darting rays of the sun as it surfaces and surveys the airy world above the waves. On the sand banks between the reefs, marine snails or gastropods and bivalves make their living on the sandy substrate. Cretaceous rocks form reservoirs for the entrapment of oil and much exploration has taken place along the South African coastlines since 1965. Although much of the drilling has been unfruitful in terms of oil, the borehole data has been highly instrumental in understanding Cretaceous geology. Gas reserves were found off Mossel Bay and these are currently being exploited, while ongoing exploration for oil continues on the continental shelf.