Passage 9.39.10
τοῦ δὲ οἰκοδομήματος τούτου τὸ σχῆμα εἴκασται κριβάνῳ· τὸ δὲ εὖρος ἡ διάμετρος αὐτοῦ τέσσαρας παρέχοιτο ἂν ὡς εἰκάσαι πήχεις· βάθος δὲ τοῦ οἰκοδομήματος, οὐκ ἂν οὐδὲ τοῦτο εἰκάζοι τις ἐς πλέον ὀκτὼ καθήκειν πηχῶν. κατάβασις δὲ οὐκ ἔστι πεποιημένη σφίσιν ἐς τὸ ἔδαφος· ἐπειδὰν δὲ ἀνὴρ ἔρχηται παρὰ τὸν Τροφώνιον, κλίμακα αὐτῷ κομίζουσι στενὴν καὶ ἐλαφράν. καταβάντι δέ ἐστιν ὀπὴ μεταξὺ τοῦ τε ἐδάφους καὶ τοῦ οἰκοδομήματος· σπιθαμῶν τὸ εὖρος δύο, τὸ δὲ ὕψος ἐφαίνετο εἶναι σπιθαμῆς.
The shape of this building resembles that of an oven; its breadth across the diameter might be estimated at four cubits, while the depth of the structure one would judge does not extend farther than eight cubits. They have not made a built-in descent to the floor; instead, whenever a man comes to visit Trophonius, they provide him with a narrow and lightweight ladder. Upon descending, there is an opening between the floor and the structure, two spans wide, and appearing to be one span high.