There is a memory in the dirt at the bottom of the sea. It is in the number of different sorts of small dead animals which we can use to find out how warm or cold the sea was in the past. There are different approaches for doing this. All the approaches have problems if things are more like things near to them in space than expected. If this happens, our guess of how warm the sea was can appear to be much better than it really is. Some often-used approaches are much worse than others when this happens. This problem is hard to wipe out but ignoring it is not a good idea.