Learn Russian: Туристы лазили по ущелью, пока не заблудились. - The tourists were climbing in the gorge until they got lost.

лазили is the multidirectional (indeterminate) verb of motion. It shows that the tourists were climbing all around or back and forth in the gorge, rather than moving in a single, continuous direction toward a specific destination, which would require the unidirectional verb лезли.

Why is the preposition по used with the dative case (ущелью) here?

When paired with a multidirectional verb of motion like лазили, the preposition по plus the dative case expresses movement around, along, or all over a surface or area. It emphasizes that they were climbing all over the gorge, not just going into it.

Why is there a не in пока не заблудились if they actually did get lost?

In Russian, when пока means until and is followed by a perfective verb that brings an ongoing action to a halt, it must be paired with не. You can think of the literal logic as the tourists were climbing around as long as they had not yet gotten lost.

I sometimes see лазали instead of лазили. Are they the same?

Yes, they mean exactly the same thing. They come from two different infinitive forms of the same multidirectional verb: лазить and лазать. While both are common in everyday speech, лазить is generally considered the stricter literary standard, which gives us the past tense лазили.