Here's the good news: The crystal-studded stalactites (the ones hanging from the ceilings like icicles) and stalagmites (the ones rising from the cave's floor) are spectacular, in shades of rose, green, amber, black, blood red, and purple.
The bad news: These caves, a popular vacation destination for Greeks, are mobbed in summer, so try to arrive as soon as they open. If you get here later in the day, buy your ticket immediately; usually visitors are taken in some kind of order. As wonderful as they are, these caves are not recommended for the claustrophobic. Guided tours through the Glyfada Cave take about 30 minutes. The largest of the cave's passageways is 100m (328 ft.) long; and although the trail is lit with multicolored lights, you are well aware that you are far underground. Guides pole small boats (each holding up to a dozen passengers) past the strange formations and call out the stalactites' and stalagmites' nicknames (ranging from the reverential to the obscene). The guides also delight in warning passengers not to trail their fingers in the cool subterranean waters that reach depths of 20m (98 ft.): Giant eels are rumored to live just below the surface. Visitors usually disembark from the boat for a short excursion on foot through a slippery segment of the cave. Greek women do this in high heels and sling-back sandals; I prefer sensible rubber-soled shoes or sandals.
The Pirgos Dirou Caves were discovered in 1955 by a dog that crawled through a hole into the caves and returned several days later coated in red clay. Fortunately, its owner, spelunker Anna Petroclides, was curious about the red clay and followed her dog when it next set off on explorations. What she found was a vast network of caves, of which some 5km (3 miles) have now been explored. The caves themselves are impressive, but what has made them famous is the Paleolithic and Neolithic remains found here. The Pirgos Dirou Caves are one of the oldest inhabited spots in Greece, and the pottery, bone tools, and even garbage found here have shed light on Greece's earliest history.
Before you leave Pirgos Dirou, see if the small Neolithic Museum (tel. 27330/52-233), with displays of artifacts found in the caves, is open. Admission is 2€.