The ancient name of the piazza, still very common today, originates in the large exedra of the baths of Diocletian which gives the piazza its shape. The porticos around the piazza , built in 1887-98 by Gaetano Koch, were actually in memory of the ancient buildings on the same sites, whilst the basilica of Santa Maria degli Angeli e dei Martiri on the piazza is based on a wing of the baths (with its architect Michelangelo using the tepidarium as one of the wings of its spacious Greek cross plan).
The naiads represented are the Nymph of the Lakes (recognisable by the swan she holds), the Nymph of the Rivers (stretched out on a monster of the rivers), the Nymph of the Oceans (riding a horse symbolising of the sea), and the Nymph of the Underground Waters (leaning over a mysterious dragon). In the centre is Rutulli's Glauco group (1911/12), symbolizing the dominion of the man over natural force and replacing a previous sculpture.
References
Touring Club Italiano, Roma, collana L'Italia (red guide), Roma 2004
Willy Pocino, Le curiosità di Roma, Newton & Compton Editori, Roma 2004, ISBN 8854100102