How to say something is under-explored?

I believe the word you're looking for is underdeveloped:

2 : having a relatively low economic level of industrial production and standard of living (as from lack of capital)
// underdeveloped nations

// Founded in 1989 with an initial $50,000 investment, Chesapeake focused on drilling in underdeveloped areas of Oklahoma and Texas.
— CBS News, "Chesapeake Energy, Oklahoma "fracking" pioneer, files for bankruptcy," 29 June 2020

This would be the before state of something that is now "on the rise" in terms of attention and, well, development.

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Of course, if you're really talking about exploration, the actual word would be unexplored, and it would refer to parts of an area: "The area remains largely unexplored."

neglected is also a useful word to describe this. It signifies that the theme exists but has not hitherto attracted much attention. In this way it differs from undiscovered, which does not suit your purpose. Here is a good example: https://pubmed.ncbi.nlm.nih.gov/16332979/

@Anton has a good answer, supported by an example, but in practice it may be more politic to avoid a term that may be taken to reflect badly on others in the field. The banner on that link gives an alternative that one could use:

Emerging

As in:

“Covid-19 is an emerging…”

(But for goodness sake, don’t copy “situation”. “Area” is fine.)