The present article discusses Piranesi's use of the ancient topos of locus horridus on the example of his four Grotteschi. The analysis of particular ingredients of Piranesi's loci horridi has shown that, in order to understand them, the onlooker must be not only familiar with the Greek and Roman mythology and literature, but also, must have the patience to detect even those details which, despite being too tiny or obscure to be noticed at the first glance, are crucial to the meaning of the whole picture. In his sophisticated game with the onlooker, Piranesi employs optical illusion and pareidolia. The article also reveals that the aesthetics of the Grotteschi consists, to a large extent, in bringing together motifs that originally belong to the opposite realms: that of a locus horridus and that of a locus amoenus. As a result of such a fusion, the locus horridus becomes even more frightening and hostile, whereas the traditionally 'idyllic' elements acquire a sinister hue.