Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's.

But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity.

[0] https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...

[1] https://en.wikipedia.org/wiki/Windcatcher

[2] https://en.wikipedia.org/wiki/Yakhch%C4%81l

The problem here is the density. Air conditioners are rated in tons of cooling, with even the small ACs rated at at least 1 ton a day. So even a small house can easily melt through its weight in ice during summer.