CALIFORNIA—On Saturday, March 7, the Coliseum in Rome was once again defaced—and this time the vandals were two California women, ages 21 and 25.

Constructed under the Roman emperors Vespasian, and succeeding him, Titus, the Coliseum, which is also known as the Flavian Amphitheatre, accommodated medieval entertainment such as gladiatorial contests, mock sea battles, animal hunts, executions, battle re-enactments and Classical mythology theater to audiences of up to 80,000 spectators.

The tourists, whose names have not been released, left their tour group on March 6 and allegedly used a coin to etch a 20-centimeter-high engraving of their initials, the letters “J” and “N”, into a western wall on the second floor of the famed landmark. The tourists took selfies of the damage soon before they were arrested.

A tour guide who noticed the vandals called police, who immediately detained the pair and cited them for “aggravated damage to a building of historic or artistic interest,” according to the Italian news agency, ANSA.

The Italian officers were quoted saying that “[The tourists] didn’t realize it was such a serious thing.”

The women will appear in front of a judge and face a hefty penalty for their actions.

The Coliseum in its current state serves as a major tourist attraction with millions of paying tourists coming each year to view what is considered one of the greatest works of architecture and engineering. Recognized by the United Nations Educational, Scientific and Cultural Organization as a World Heritage site, the ancient concrete and stone amphitheater is also a current site of worship for Roman Catholics.

Last November, Roman authorities handed a 42-year-old Russian tourist a €20,000 ($21,089) penalty for carving the letter “K” into one of the bricks. He, too, was charged with aggravated damage.

Other incidents of vandalism at the Coliseum in 2014 involved two Australians—a father and son—and two teenagers, a Brazilian and a Canadian.