Tham Khao Maikaeo
Monsoon rains pounding away at limestone crevices for millions of years have created this complex of caverns and tunnels. There are cathedral-size chambers, dripping with stalactites and stalagmites, tiny passages you have to squeeze through on hands and knees, and even a subterranean pool. A local family runs hourly treks to the caves (with headlamps). The full trip takes two hours; sensible shoes essential. It's signposted off the main road from Hat Khlong Tob to the east coast. Phone ahead for timings.