You may have to dig deeper to get undisturbed earth for your foundation.

There is often all sorts of junk in the ground around an old place.

I've only built infill--tearing down old homes on city lots to build new ones. Since the new homes are always much larger and deeper than the old ones, I'm always facing uncertainties, but never enough to order test holes.

So far, the only big surprise was finding a buried fuel tank half-filled with oil left over from before the 1955 home we tore down was built. That was in the late '80s. Nowadays, under environmental rules that surprise could easily cost us many thousands of dollars: environmental tests, removal of contaminated soil and replacement.

A former building partner was hired to build a custom home on a riverfront lot. When he excavated, he found it was actually an unmarked old dump site that had to be removed and filled with dozens of loads of clean gravel before he could build on it.

Sometimes opening an excavation resembles lifting the lid on Forest Gump's box of chocolates.