Dominica's boiling lake, known simply as Boiling Lake on the island, is the world's second largest hot spring. The world's largest is Frying Pan Lake in New Zealand. Boiling Lake is located roughly in the middle of the island and can be accessed from many directions, but the easiest and most popular is via the Titou Gorge.
I don't have pictures of the Titou Gorge, but if you watch Pirates of the Caribbean you'll see it featured in that movie. It's a short, flooded chasm with a waterfall at the narrow end and a shallow pool at the open end. Inside it moss covered walls arch over and up about 30 feet. It's a fun, refreshing swim up the gorge after a hike to the lake.
The hike itself is roughly 6 miles one way, but it's hard to get a proper estimate without equipment and finding a topographic map of the island has proven impossible. Along the way one crosses the Breakfast River, heads up the 3000' Morne Nichols, and descends into the Valley of Desolation, a volcanically active bed of hot springs near the lake.
I went on this hike three times while on the island and it's one of my favorites here. It's a well-rounded hike - some hills, some jungle, and a lot of unique and beautiful views. It's the second easiest of my four favorite hikes on the island and definitely family friendly if everyone is in decent shape. Be warned though, it can be miserable on a hot, cloudless day so do bring plenty of water if you want to try this one for yourself.