A buoyancy engine, including an upper casing, a lower casing, and a manifold separating the upper and lower casings. The manifold has internal and external control means, that control amounts of liquid entering and leaving the manifold and casings through a primary route and an alternate route. There is a variably ballasted power piston within the upper casing, into and out of which liquid ballast can be pumped to change its buoyancy. There is a liquid discharge piston within the lower casing, that can be coupled to the variably ballasted power piston to form a primary pump. There is also a backup pump that can be used when the primary pump fails. A mechanical slide valve controls the entrance and exit of liquid from the manifold through the primary route.