Passage 2.37.5
εἶδον δὲ καὶ πηγὴν Ἀμφιαράου καλουμένην καὶ τὴν Ἀλκυονίαν λίμνην, διʼ ἧς φασιν Ἀργεῖοι Διόνυσον ἐς τὸν Ἅιδην ἐλθεῖν Σεμέλην ἀνάξοντα, τὴν δὲ ταύτῃ κάθοδον δεῖξαί οἱ Πόλυμνον. τῇ δὲ Ἀλκυονίᾳ πέρας τοῦ βάθους οὐκ ἔστιν οὐδέ τινα οἶδα ἄνθρωπον ἐς τὸ τέρμα αὐτῆς οὐδεμιᾷ μηχανῇ καθικέσθαι δυνηθέντα, ὅπου καὶ Νέρων σταδίων πολλῶν κάλους ποιησάμενος καὶ συνάψας ἀλλήλοις, ἀπαρτήσας δὲ καὶ μόλυβδον ἀπʼ αὐτῶν καὶ εἰ δή τι χρήσιμον ἄλλο ἐς τὴν πεῖραν, οὐδὲ οὗτος οὐδένα ἐξευρεῖν ἐδυνήθη ὅρον τοῦ βάθους.
I saw also a spring called that of Amphiaraus, and the lake named Alkyonia, through which the Argives say Dionysus descended into Hades to bring back Semele, and that Polymnus showed him this descent. Alkyonia has no known bottom, nor do I know of anyone who by any contrivance whatsoever has succeeded in reaching its extremity. Even Nero, after preparing ropes many stadia long, fastening them together, and attaching lead weights to them—as well as other things useful to his experiment—was unable to discover any limit to its depth.