The immense Baths of Diocletian are in ruins, but parts of them are still visible in re-use as churches and museums centered on the Piazza della Repubblica.
Above -- Plan of the baths over Rome's current street plan. North is at the upper left corner and south at the lower right. The main entrance to the baths was at the top of the diagram.
Above -- Red areas are still visible or are replicated in current structures. The large semi-circular red feature at the bottom of the diagram (the exedra) serves as the base for he two large Commercial Palazzi (built by Gaetano Koch, from 1896-1902) on Piazza della Repubblica.
Above -- The green circular feature (lower center of the diagram) is the current fountain in Piazza della Repubblica: fountain basin by Alessandro Guerrieri in 1901; Glaucus and Niads sculptures by Mario Rutelli, 1901-1911. Directly above the fountain in the diagram is the entrance to S. Maria degli Angeli e Martyri that occupies the central area of the remaining structure. The smaller circular feature at the lower left of the diagram is now the church of S. Bernardo alle Terme, and the corresponding feature in the lower right is diagonally across from the National Museum in Palazzo Massimo. The oddly shaped almost symmetrical green features along the upper right edge of the picture are the parking areas of the Piazza del Cinquecento in front of the Termini Rail Terminal.