Rummu quarry, a unique former prison and limestone quarry in Estonia, is best visited in summer (June-August) for swimming and water activities in its clear, turquoise lake, though spring and autumn offer quieter, picturesque exploration. Accessibility is primarily by car, with parking available, though public transport to nearby towns followed by a taxi or walk is possible; the site has uneven terrain and limited facilities. Visitors can explore independently, but guided tours are recommended for diving/snorkeling in the submerged 'forest' or for deeper insight into the historical prison ruins. Expect a small entrance/parking fee (a few euros), with additional costs for guided activities like diving (e.g., €50-€100+). A typical visit lasts 2-4 hours for sightseeing and swimming, extending to a full day for more involved activities. While visually stunning, visitors should exercise caution due to abandoned structures, uneven ground, and deep water.