It is because you are going "below" sea level.
It is like saying going below zero, since sea level is at zero elevation.
If you were to go "below" zero, you would get a negative number, the same with going below sea level. Since you are going below 0 [elevation], you write your elevation below sea level in a negative value.
Roydon Freeman has written: 'Sea travel' -- subject(s): Ocean travel
Stuart C. Cumberland has written: 'The Queen's highway from ocean to ocean' -- subject(s): Accessible book, Description and travel
If it is on a map for ocean travel it would mean degrees
Rewa has written: 'Molly May on the high seas' -- subject(s): Fiction, Ocean travel, Orphans
Jason Vogel has written: 'Hot spots' -- subject(s): Guidebooks, Ocean travel, Resorts
In clubpenguin, penguins can't travel the ocean...........
Helmut Cauer has written: 'Fernweh auf Schiffsplakaten' -- subject(s): Ocean liners, Travel posters, Posters
Joseph Foos has written: 'The highway of all nations' -- subject(s): Canals, Interoceanic Canals, Ocean travel
Christian Xadow has written: 'Whisky, Haut & [und] Serum' -- subject(s): Biography, Ocean travel, Ship physicians
John Wm Macchi has written: 'Choosing the right cruise for you' -- subject(s): Cruise ships, Ocean travel
Douglas E. Darbyshire has written: 'In time for lunch' -- subject(s): Emigration and immigration, History, Ocean travel
Counting Numbers, Whole Numbers. You can't use Integers as that includes Negative numbers and there isn't a negative number of dolphins.