The confusion of the MSS is well justified; something has gone very wrong here. Even if ‘horrendis... profundis’ could be plausibly construed, the repetition ‘horrendis... horrendi’ is impossibly clumsy, and it seems obvious that one or the other does not belong here. I suggest that the interloper is the ‘horrendis’ of line 286, which probably derives from a simple eye-skip to ‘;horrendi.sociis’ below. The likely corollary is that the correct reading at the end of the line is ‘profundi’, later altered in an attempt to accommodate the intrusive ‘horrendis’. This approach would seem to be confirmed by the frequency of the clausula ‘stagna profundi’ in the Latin hexameter (cf. Lucan 2.571, 8.853, 9.305, Sil. 7.282, 378, 10.590, Avien. Arat. 991, Claud. 8.596, Coripp. Ioh. 6.23).