Story highlights A bus still takes people from Beirut to Raqqa in Syria, now the stronghold of ISIS

Men are advised to grow long beards before they travel and wear loose pants

Passengers scrub their mobile phones of anything that ISIS forces may not like

Beirut, Lebanon (CNN) It's a pretty normal bus -- windows slightly cracked, dust, the occasional button missing on the dashboard. But when its passengers say they take it knowing they could be on a one-way ticket to death, they aren't exaggerating.

From the dark and dank underpass that is Charles Helou bus station in central Beirut, leaves the bus to Raqqa. It has done so for years, but now that Raqqa is the capital of ISIS' self-declared caliphate, the bus crosses the most dangerous border in the world. And people do pay to get on it.

In a 24-hour journey, it travels from Beirut, across the border to regime-held Damascus. Then it heads to Palmyra, held by ISIS, before moving north toward Raqqa.

The nine passengers we met were adamant about two things. First that ISIS would most likely let them in to Raqqa. That suggests they know someone there, and they didn't want to go into details. Second, nobody wanted to have their face filmed or name used.

The fear was overpowering and that permeates exactly how you get ready for the trip.

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