The northern French coastline offers a smorgasbord of hiding places for smugglers. Trees give good protection from surveillance planes, World War Two bunkers are ideal accommodation for passengers, and sand dunes and scrubland hide boats and slow patrols.
Equipment has helped. UK-funded drones, equipped with thermal-imaging cameras, have changed the game for gendarmes here, spotting people even under tree cover.
Specialist buggies built for sand dunes - another contribution from the UK - have helped units get there faster. But the last stretch to where a boat is being prepared for launch often must be covered on foot - through soft sand and thickets of thorny shrubs, sometimes chest-high.
I've seen officers emerge from the chase with long deep scratches on their bodies - the thorns having torn through both uniform and flesh.
It's not hard to hear a patrol wading through that scrub. By the time gendarmes arrive, there's often just an abandoned boat and some petrol left behind.
The smugglers know what they're doing.
There is, the UK government says, no silver bullet. But the resources ploughed into this coastline, year after year, are having some effect: 30,000 crossings attempts have been intercepted so far this year.
And this latest deal contains two key messages about what's working - operationally and politically.
Firstly, there's more investment for equipment and patrols, with the goal of doubling the proportion of crossings that are intercepted.
And secondly, there's a new agreement that France will host British officers in command centres on French soil, to share information and help direct resources.
Sources close to the negotiations also tell me that France has also offered to allow British officers to come out on patrol with French unit as observers.
And Tony Smith, former Border Force director-general, believes it's a step in the right direction.
It might reduce the flows if it works, but it's not going to stop the boats altogether.
"We haven't had very much control in the past, about exactly how the French deploy the resources we're paying for," he told me. "This new agreement means that we have now got operational and tactical liaison going on about deployments."