Filling gaps in a concrete floor
Self-levelling cement can be quite expensive. You might want to fill the deep gaps to the level of the rest of the floor, and then top (the whole mess) with a self-levelling product if that's your desired end-result. If you are just &quot;cleaning up the basement&quot; and don't have some plan in mind that calls for a very flat floor, you might also consider troweling on &quot;sand mix&quot; to make a finished troweled surface after you fill the gaps, without getting too bent about self-levelling (and beware if the overall floor is not actually level when you let self-levelling cement loose...)
There's a case to be made for demolishing the whole mess and starting from scratch if what you see is &quot;dirt&quot; (quite likely given the workmanship and age) rather than a nice stone base, but if it isn't moving or otherwise causing issues, that might be a lot of work and expense for not much real improvement. So I don't consider it a strong case unless you have something particular in mind where that seems like it might be worth the bother.