The Pyramid on the Sea Floor - S1.E7

Upon seeing an odd looking formation from space, Harlock's crew heads to the Bermuda area. Upon investigating a pyramid built by the Mazone on the ocean floor, they learn that the Mazone are actually a form of plant life. Learning that the Mazone are hidden across the globe and that the government has not heeded their warnings, the crew sets off on a search and destroy mission. Learning that there are the Mazone near Mayu as well, they set out to kill them.