It could be done, I think. But most underground areas aren't entirely stone - there's lots of ores, air and other impurities in the ground so the results wouldn't be that good (maybe a 3% ram reduction for 25% less speed).
Actually, water might be a good candidate. Wouldn't have the biggest impact, but oceans are quite big.
Actually, water might be a good candidate. Wouldn't have the biggest impact, but oceans are quite big.