pothos.org

Trajan at the beginning of spring (AD 116) hastened into the enemy's country. And since the region near the Tigris is bare of timber suitable for building ships, he brought his boats, which had been constructed in the forests around Nisibis, to the river on waggons...And the Romans crossed over
and gained possession of the whole of Adiabene. This is a district of Assyria in the vicinity of Ninus (Nineveh); and Arbela and Gaugamela, near which places Alexander conquered Darius, are also in this same country.

Here, moreover, Trajan saw the asphalt out of which the walls of Babylon had been built. When used in connexion with baked bricks or small stones this material affords so great security as to render them stronger than ant rock or iron. He also looked at the opening from which issues a deadly vapour that destroys any terrestrial animal and any winged creature that so much as inhales a breath of it.

Then he came to the ocean itself (Persian Gulf), and when he had learned its nature and had seen a ship sailing to India, he said: "I should certainly have crossed over to the Indi, too, if I were still young." For he began to think about the Indi and was curious about their affairs, and he counted Alexander a lucky man. Yet he would declare that he himself had advanced farther than Alexander, and would so write to the senate ..

Trajan learned of this at Babylon; for he had gone there (on his way southward) both because of its fame - though he saw nothing but mounds and stones and ruins to justify this - and because of Alexander, to whose spirit he offered sacrifice in the room where he had died.