10,000 years of Cascadia earthquakes

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If there isn't water under the slab then it's a combination of moisture and sanded grout. Sanded grout absorbs water and swells all the grout lines creating expansion. When the grout expands it squeezes all the tiles together. The resulting pressure causes the areas with the highest volume of water to squirt upward resulting in those little mini volcanoes you see every now and again. Try opening all the windows and let the place dry out for a day or two. I know this because I've opened up many s