Sometimes what happens is an old industry dies out or matures, say heavy manufacturing, the dumping happened long before there were strict anti-pollution regulations, but the area is otherwise nice and in an central or attractive location, so housing gets built.<p>Then people learn heavy metals and other chemicals which leached into the soil are unhealthy.  Some times people know about it and a superfund gets declared, other times it was forgotten and people find out many years later.<p>New developments in Mountain View[1], for example, sit on top of superfund sites.  Some of which cannot be alleviated but rather &quot;managed&quot;.<p>Hunter&#x27;s point in Sf is another example.  That was a navy shipyard with lots of lead heavy metals and other things from WWII war effort --but people eventually built out housing there.<p>[1]<a href="http:&#x2F;&#x2F;www.theguardian.com&#x2F;commentisfree&#x2F;2014&#x2F;mar&#x2F;19&#x2F;google-mountain-view-toxic-waste" rel="nofollow">http:&#x2F;&#x2F;www.theguardian.com&#x2F;commentisfree&#x2F;2014&#x2F;mar&#x2F;19&#x2F;google-...</a>