Lago Cardiel is much larger than I expected; we can barely see the opposite shore. This side is ringed by low hills and opens up like an apron, broad and gently sloping. As soon as we come through the hills, we notice well-defined former shorelines, which look like soap residue rings around a bathtub.Shorelines make great natural roads, just as glacial outwash fields are apparently well-suited for airport runways because they are flat and drain well. JFK is built on a glacial outwash plain, Mike tells me.We drive along until we spot an outcropping of basalt rock and carbonate tufa. Littered over this surface are the tiny white shells of dead snails which once lived near the shore. Their presence indicates that the lake shoreline must have been this high at some point. The shells must have been left when the lake receded.An old snail shell that was embedded in the old shoreline, in a layer of dark beach cobbles. Above the shell is the newer shoreline. Credit: Ale Borunda.The lake has grown and shrunk many times over the past 20,000 years. When it grows, it leaves behind a “bathtub ring” of residue around its edge: pebbles smoothed by wave action, carbonate tufas, the remnants of the things that lived at the shore. There are at least three distinct bathtub rings (each of which represents a time when the lake surface was higher than it is today) that we can see in a Google Maps printout we’ve been carrying around. But Scott Stine, a researcher who did his Ph.D thesis on Lago Cardiel, has identified other shorelines beyond the three we can see now.