Published in TV21 (New Series) Annual 1972.
An undersea research probe operating more than 500 feet below the surface suddenly is engulfed by a huge black shadow and goes offline. A hundred miles away, Johnny (sic) Webb is testing S.N.O.R.K.E.L.'s latest vessel, the ultra-fast Manta. Webb, his partner Mizen, and the Manta will be used to investigate the trouble.