ABSTRACT: The Greek Campanian architect L. Cocceius Aucto, active in the late Roman republican age, is believed to be the principal artificer of the complex of infrastructures serving the harbour of the Roman fleet, the Portus Julius, in the Phlegrean Fields east of Naples. They include three outstanding road tunnels that have been excavated in few years, starting on 39 B.C. under the impulse of Octavianus, who will soon become the first emperor Augustus, and his deputy and son in law Marcus Vipsanius Agrippa. The road tunnels have been practically invented by Augustus and Agrippa, but it was Cocceius to put their program in tangible form. In fact, the three Phlegrean tunnels are the longest and most impressive ones of the whole Roman civilization and represent unprecedented engineering masterpieces. The paper reports their main features, the geotechnical problems affecting them and the planned or carried out remedial interventions.