Titus Livius (Livy), The History of Rome, Book 7
Rev. Canon Roberts, Ed.

5. In¹ this year, owing either to an earthquake or the action of some other force, the middle of the Forum fell in to an immense depth, presenting the appearance of an enormous cavern. [2] Though all worked their hardest at throwing earth in, they were unable to fill up the [3??] gulf, until at the bidding of the gods inquiry was made as to what that was in which the strength of Rome lay. [4] For this, the seers declared, must be sacrificed on that spot if men wished the Roman republic to be eternal. The story goes on that M. Curtius, a youth distinguished in war, indignantly asked those who were in doubt what [5??] answer to give, whether anything that Rome possessed was more precious than the arms and valour of her sons. [6] As those around stood silent, he looked up to the Capitol and to the temples of the immortal gods which looked down on the Forum, and stretching out his hands first towards heaven and then to the yawning chasm beneath, devoted himself to the gods below. [7] Then mounting his horse, which had been caparisoned as magnificently as possible, he leaped in full armour into the cavern. Gifts and offerings of fruits of the earth were flung in after him by crowds of men and women. [8] It was from this incident that the designation ‘The Curtian Gulf’ originated, and not from that old-world soldier of Titius Tatius, Curtius Mettius. [9] If any path would lead an inquirer to the truth, we should not shrink from the labour of investigation; as it is, on a matter where antiquity makes certainty impossible we must adhere to the legend which supplies the more famous derivation of the name.