An aurora was described by the Greek explorer Pytheas in the 4th century BC. Seneca wrote about auroras in the first book of his "Naturales Quaestiones", classifying them, for instance as "pithaei" ('barrel-like'); "chasmata" ('chasm'); "pogoniae" ('bearded'); "cyparissae" ('like cypress trees'), and describing their manifold colors. He wrote about whether they were above or below the clouds, and recalled that under Tiberius, an aurora formed above the port city of Ostia that was so intense and red that a cohort of the army, stationed nearby for fire duty, galloped to the rescue. It has been suggested that Pliny the Elder depicted the aurora borealis in his "Natural History", when he refers to "trabes", "chasma", 'falling red flames' and 'daylight in the night'.