Lexitechnia

Pliny on dangerous water (Naturalis Historia 31.19):

Et haec insidiosa condicio est, quod quaedam etiam blandiuntur aspectu, ut ad Nonacrim Arcadiae, omnino nulla deterrent qualitate.

There is also this treacherous circumstance, that some [poisonous waters] may even be enticing in appearance, as at Nonacris in Arcadia; they do not deter with any distinctive quality at all.

There is a waterfall called Styx in that area, which in Modern Greek is also called Μαυρονέρι (Mavroneri ‘Blackwater’), which was at one time the tallest waterfall in Greece. The ancients believed its water was not only poisonous, but could hardly be collected because it would break or dissolve just about any material, save for the hooves of horses. They also relate that Alexander the Great was perhaps poisoned by this water. Apparently superstitions still remain about it; I couldn’t determine how poisonous or corrosive the waters are known to be, but I did find a couple of pictures on Flickr:

• Mavroneri waterfall
• Another view of the Styx

[For qualitas.]