The constant flow of water into and out of and over stone demands that the Playboy Mansion use a sophisticated filtration system. 20,000 gallons of water tread over the estate, their streams contained in underground pipes, following a special blueprint, filed in a nondescipt folder in the architect's office, finally meeting in a shack out of view. There, bulbous fiberglass filters are responsible for the entire system: the waterfall, the Japanese koi pond, the pool, and, on the obverse side, the grotto.
The water in the grotto would tip a boat. Since it's a structure of four units posing as one - four jacuzzis with rugged looking steps between them- it has changeable currents, and one person may create waves for another several jacuzzis away. But is nonetheless tucked away, surrounded by rock.