If you’ve been following Skipper Wilson’s progress in the Vendée Globe, you’ve probably noticed that periodically he talks about having to go through an ice gate.

An ice gate isn’t a gate at all. In fact, it’s not even an actual thing. Instead, it’s an imaginary line drawn between two points 445 miles apart. At each of these imaginary lines, the boats must pass north of at least one of these endpoints. (You can see in this image the different ways a boat can achieve this goal. The different colored lines are possible routes around the ice gate, which is the solid black line connecting the two circles.) The race creators came up with the idea to keep the sailors and their boats safe from icebergs.

Staying north of these points help keep the racers away from the dangerous southern waters, where summer temperatures are warming the freshwater glaciers and ice shelves. This causes pieces of them to break off (in the same way that an icicle might fall off a snowy roof as the weather warms up), drop into the ocean, and float away in the ocean’s currents. These pieces are called icebergs.

Icebergs range in size from relatively small (less than 16 feet long or high) to the huge (some are more than 250 feet tall or 660 feet long). Obviously sailors can see to stay away from something really big, but smaller icebergs pose bigger problems. This is especially true because although icebergs float, a lot of their mass is below the surface of the water. In fact, only 1/8 of an iceberg is visible above the surface of the water. So while an iceberg might look small and like a boat could move around it easily, it could stretch out quite a ways under the water, where a boat could crash into it.

Scientists monitor where larger icebergs can be found using satellites, buoys, and radar. Because ice can melt and break up, though, race organizers want to keep the sailors far from where icebergs are likely to be found.

Captain Murray Lister, a friend of Skipper Wilson, writes a little about icebergs in this week’s essay on sitesALIVE!

Skipper Wilson is approaching the final gate, the Eastern Pacific Gate, of the eight that the Vendée Globe officials announced for this year’s race and should pass through it this weekend or early next week. Read his latest log here.