This was posted o a fossil forum by a collector from Florida Venice Gulf
the rock is hollow with columns in the middle joined only by the top and bottom
looking for help with identification
Difficult to say without having it in the hand and able to examine in thin section.
Going just on the photos, I can guess it might be a piece of rock which used to include mineral veins, which have dissovled leaving the cavities you see now (and hence the 'columns' between the cavities). Mineral veins come in many geometries: fat/thin, planar/curved/irregular, intersecting/parallel, and individually they can be just the shape seen in your specimen (Google 'tension gash array'). However, as an array, such tension gashes should be inclined to their bounding envelope - not at right angles like yours.
...Consider that as my thinking out loud, rather than an ID.
It might be man-made.
If you get an ID on the Florida forum, please post here.
If not, you'll have to take it to a museum or university geology dept and ask nicely if somebody will help you ID it. They'll probably want to cut it with a rock saw and make a thin section to analyse under the microscope. They will/should also ask for details of where it was found.