Twenty-two metres below Israeli territory, I could just about stand up. In the gloom I could see Arabic writing on the concrete walls, and an arched ceiling.
A tunnel, more than a mile long, begun in Gaza and ending up 300 metres inside Israeli territory. It was some piece of work. Built to last, a long way down where the earth is more stable, and equipped with electricity and even a phone line.
Israel's prime minister calls it a "terror tunnel," and accuses Hamas - the Islamist organisation which controls Gaza - of building it. A Hamas military commander responded defiantly to the Israeli army's discovery of the tunnel this week. The minds which built the tunnel, he said, will build dozens more.
Israel has good reason to be fearful of that prospect. In 2006, Hamas militants used a tunnel to get into Israel and attack an army patrol. They killed two soldiers and took another hostage. Gilad Shalit was held hostage in Gaza for five years, and only released after Israel agreed to release more than a thousand Hamas prisoners in return.
The head of the Israeli Defence Force, Benny Gantz, voices an even bigger nightmare scenario - that "an explosives-laden tunnel would be detonated under a kindergarten near the border".
Such an event would mean war, and the snuffing out of the latest attempt at reviving the peace process here. A peace process from which Gaza is, in any case, far removed. Hamas refuses to recognise Israel, refuses to renounce violence and is regarded by the United States and the European Union as a terrorist organisation.
Tunnels are further evidence of Hamas' isolation. While the passages under the Israeli border may have been designed for purely aggressive purposes, the hundreds of tunnels which have been dug beneath Gaza's southern border with Egypt have been vital supply lines. Everything from fridges to fuel has passed through them - the things which make life bearable for the 1.7 million people who live in Gaza.
However, Egypt's new military government is convinced that those tunnels are being used by terrorists carrying out attacks in the Sinai peninsula, so Egypt's army has been destroying them. They blew one up just last weekend - leaving Gazans and their government more isolated than ever.
Back on the Israeli side of the border, the local commander, Brigadier General Mickey Adelstein, tells me he knows of the existence of several more tunnels being dug, and that his surveillance teams are watching them, waiting for his order to close in. His men are trained to fight in tight spaces. They eat their meals in the shade of their bulldozers. The ceasfire feels very fragile here.