It's a VTOL pit. The one I know about was just a big hole in the ground
with a grid over it flush with the surface of the runway it was located
in (and a waterproof lining and a little pump to stop it becoming a
swimming pool). The intent was (and is) to remove ground effects, which
can cause control and engine thrust/surge-margin problems. On the basis
that your new VL-capable jet has enough grunt to hover without ground
effects (which can also be used to boost performance, as in the Harrier
II), it's good to be able to test VTO and/or VL without the potential
problems ground effects can cause. I know a tale of someone who tried to
drive a hovercraft over one. Not a good idea.

hobo wrote:
http://www.codeonemagazine.com/photo...y/photo12.html


In this photo the JSF is hovering over what looks like a giant metal
grill. Is this a special test site for testing JSF hovering? I imagine
that such a grill would allow you to simulate higher altitude hovering
without the dangers.