How can you see lost buildings from the air?

If you go to the Roman fort at Caistor St Edmund you'll see a picture taken from an aeroplane, of the lines in the grass which shows where the roads and buildings of the town used to be.

These lines are usually called crop marks. Even if a ditch was dug two thousand years ago and then filled in, it will still hold more water than the un-dug soil next to it. When crops like  wheat are planted over the site of an old ditch their roots can get more water so they grow and ripen quickly and that makes them look different to the rest of the crop. From the air they look darker. 

And if hard areas were made to support buildings and roads, these stay dry for centuries, and crops grow less well and also look different - these area appear lighter from the air and are called 'parch' marks. 

In very hot summers these marks show up very well, and archaeologists can work out what kind of buildings used to stand there!