Kurdish People’s Protection Units, or YPG, discovered the half-completed tunnel after they captured the border town of Tal Abyad last week, spokesman Redur Khalil said.

He said it wasn’t clear whether it was eventually meant to lead into Turkey as the tunnel splits into two different directions at one point.

A tunnel linking Turkey to ISIS-held territory might offer some insight into the many ways in which Muslim radicals from around the world have slipped across the Turkish border to reach the fanatical group, whose territory straddles Syria and Iraq.

The YPG captured Tal Abyad last week from ISIS, cutting off a vital supply line to the group’s self-proclaimed capital, Raqqa.

The Kurdish advance prompted thousands of Syrians to flee the fighting and go across the border into Turkey, but hundreds began moving back into Syria after Turkey reopened a nearby border crossing Monday.

Local authorities in Sanliurfa Province confirmed Monday that the official border was now reopened and that Syrians were beginning to return. The Turkish news agency DHA put the number of returnees at 500.