Rome is one of the world's prettiest cities yet lurking behind this beauty is a host of grim, gross or spooky attractions.

Tourists can visit cliffs haunted by murdered souls, the basilica which hid a bizarre cult, the church which exhibits a celebrity skull, the historic site scarred by Mafia violence, and a trailblazing sewer. Here are five of the Italian capital’s darkest sites.

1. A famous cranium

I find it simultaneously gruesome and beautiful. Before me, inside Rome’s 6^th century Basilica di Santa Maria in Cosmedin, is a cracked and discoloured human skull embellished by colourful flowers and encased in a golden reliquary. While its presentation is elegant, far more casual is the painted white label on its forehead which reads: “S. Valentini M.”.

Surprisingly, that’s the only indicator I can see here that these are the remains of Valentine, perhaps the world’s most famous Saint. Every year on February 14, couples from Sydney to Stockholm celebrate their love on Valentine’s Day, an ancient festival with cloudy origins.

Similarly uncertain is whether the skull I’m staring at actually belonged to St Valentine, an Italian holy man who died more than 1,700 years ago. Because across Europe there’s a dozen churches which claim to possess a physical remnant of Valentine. Historians say ancient relic traders tricked many such churches into believing they were buying the teeth, bones or clothes of famous saints. Whether it is the real deal or not, this skull is eerily interesting.

2. The weird men that gathered beneath Rome

As I collapse to my knees in Rome, I realise I’ve become enthralled by a cult. Not a group of hippies living in a compound, or a crew of pyramid-scheme fraudsters, but a bloodied gang of bull-slaughtering, gibberish-chanting oddballs who once hid beneath this city.

My fascination with the Cult of Mithras bloomed in London, where I visited the remains of their 1,800-year-old temple and learned this shadowy clan had their headquarters here in Rome. That aforementioned tumble of mine occurred outside one of their lairs, the magnificent, 2,000yo Basilica of San Clemente, just 250m east of the iconic Colosseum (pictured here).

I had been leaning down to peer through a grate into the church’s basement. That was where cult members more than 1,500 years ago performed animal sacrifices to honour Persian sun god Mithras. There were 200 such subterranean Mithras temples in Europe at one point. Now this cult lives on only through its architectural remnants, and the curiosity of odd travellers like me.

3. The precipice of death

By this point in the day I’m feeling a tad perverse, having moved from hunting a cult, to admiring a Saint’s skull, and now staring at a cliff where hundreds of people plunged to their death. This place is called Tarpeian Rock. It was an infamous execution site at Capitoline Hill, which was the ceremonial hub of ancient Rome and is now a key tourist attraction due to its array of archaeological sites.

For many centuries, starting 2,500 years ago, criminals were dragged to this clifftop via what was hauntingly known as the Stairs of Wailing. That name should be self-explanatory. Those terrified law-breakers were shoved off the cliff, with their final screams echoing across the adjacent Tiber River.

4. Mafia church

No sooner had I departed that cliff of death than I was peering at the site of an infamous car bombing. The 7^th century San Giorgio in Velabro church was shut for a long time after it was embroiled in a nationwide Mafia killing spree in 1993.

The murder of a public prosecutor in 1992 triggered Italy’s biggest-ever crackdown on its Mafia. This in turn prompted the Mob to fight back with car bombings in Milan, Florence and Rome that killed 10 people. Luckily, no one died when the explosion occurred at San Giorgio in Velabro. It was badly damaged, but has since been restored to its sublime self and stands as a testament to Italy’s ongoing battle with organised crime.

5. An intriguing sewer

What more fitting way to finish my off-colour tour of Rome than by visiting a sewer. Just a few minutes’ walk west of that mafia-tainted church, alongside the graceful old Ponte Palatino bridge, are the remains of an ancient engineering marvel.

A large stone archway, on the banks of the River Tiber, marks an entrance to the Cloaca Maxima. This was one of the world’s first sewerage systems. More than 2,000 years ago, this enclosed stone channel was built to funnel storm water from Rome’s busy forum precinct. Carried along was the waste from the forum’s connected latrines. These days the inactive sewer is one of Rome’s offbeat historical attractions.