Pamukkale, 18 km north of Denizli (map), isTurkey's foremost mineral-bath spa because of its natural beauty: hot calcium-laden waters spring from the earth and cascade over a cliff. As they cool they form dramatic travertines of hard, brilliantly white calcium that form pools.
Named the Cotton Fortress in Turkish, it has been a spa since the Romans built the spa city of Hierapolis around a sacred warm-water spring. The Antique Pool is still there, littered with marble columns from the Roman Temple of Apollo. You can swim in it for a fee.
You can spend a pleasant day at Pamukkale, exploring the extensive Roman ruins of Hierapolis, climbing the ranks of seats in the great Roman theater, touring the exhibits in the Archeological Museum, splashing along the travertines (where permitted) and even soaking in the Antique Pool littered with fluted marble columns.