There are some places so low that they are below sea level. Take Bad Water in Death Valley, for instance, it's over 200 feet below sea level. It is possible, then, as you descend deeper into Death Valley to be below sea level by different amounts. Let's say that Jill is at ten feet above sea level and that she lowers her altitude by thirty feet. What is her elevation after changing her position?
10 - 30 = -20
Next, let's say that she descends another fifty feet.
-20 - 50 = -70