Visibility of Map using a tabbed interface

I am trying to use a map in a tab system on my site. When I load the
page, then click the "On The Map" tab, the map is the incorrect size
(very small). However, it I load the page with the "On The Map" tab
selected the map is correctly displayed.

Does anyone have any thoughts on this?

Expected map Display:
http://www.tampadiving.com/sections/diving/cave_systems/system_test.asp?ID=26&directions=true&tab=3

Incorrect Map size:
http://www.tampadiving.com/sections/diving/cave_systems/system_test.asp?ID=26&directions=true

Many thanks in advance

Walter