They are not lost.
This is not a search for a buried treasure or lost keys. When you are looking for long tail keywords, they already exist in territory you have not fully explored. To define long tail keywords, these are search words that are longer than 3 words.
They are an important tool in your SEO arsenal as they are used quite often by people looking for different products, and so on. What makes them so important is that 92% of all keywords used are long tail ones.
Also, long tail keywords are easier to rank because there is little competition for them. They are also far more specific in nature. To guide you in how to find good long tail keywords here are some pointers that should help