> there's a real threat by large vessels. Which is a bit weird: Why have those pontoons on the surface in the first place? They could provide just as much buoy

> there's a real threat by large vessels. Which is a bit weird: Why have those pontoons on the surface in the first place? They could provide just as much buoyancy if they were tethered just above the tunnel itself, at a depth of some tens of meters. Or below it, pushing upwards in stead of pulling. (This would also avoid any potential problems with ice dragging them sideways in the winter.)