Below sea level is a term meaning - um - below sea level. So what does that mean anyway?
Well, believe it or not, sea level is measured in two different ways. The first and most obvious way is to measure sea level relative to the closest shoreline of the major oceans. A long time ago, this was done with gauges. Scientists would more or less drop a line into the water and measure the rope from the point it entered the water to its bottom. Over a period of months and years, an average value could be calculated for sea level in that area, and could expand to include other measured areas.
However, this way has gradually given way to new satellite technology and advanced bathymetry (the measuring of depths) techniques that now give the "mean sea level" of the major oceans on Earth. When you see most measurements of things below or above sea level today, it is based on this standardized reference.
As to be expected, the majority of land below sea level is also below the sea. The deepest point in the ocean is the Marianas Trench, which stretches down some 35,840 ft (10,924m) below sea level! Other trenches throughout the ocean exceed 25,000 ft below sea level. Here is a list of the lowest points of land on the various continents:
Africa: Lake Assal, -512 ft (-156m)
Antarctica: -8327 ft (-2538m)
Asia: Dead Sea, -1341 ft (-409m)
Australia: Lake Eyre, -52 ft (-16m)
Europe: Caspian Sea, -92 ft (-28m)
North America: Death Valley, -282 ft (-86m)
South America: Valdes Peninsula, -131 ft (-40m)