Every procedural programming language is a series of function / procedure calls, with one function / procedure calling another. It is possible to represent this sequence of calls from procedure to procedure using a call graph see Wikipedida for a starting point for call graphs. These graphs generally show the sequence of procedural calls starting at the top and proceeding to the bottom.
I quickly produced a diagram of a partial call graph for an ASP .NET MVC application. The diagram is only a partial representation for an ASP .NET MVC application because it omits things such as the initial receipt of the request by the operating system (eg. Windows), web server (eg. IIS) and various components of ASP .NET that are responsible for processing a HTTP request as it travels through the ASP .NET Request processing pipeline. These matters can can be omitted for the purposes of this discussion. Although it's worthwhile noting that they would sit at the top of the call graph, because they deal with the initial stages of handling the HTTP request, and ultimately, at some point ASP .NET ultimately invokes an action of a controller.
As you can see in the diagram I've represented the action of the controller that would be invoked by ASP .NET as an async action. On the left hand side of the call graph is a series of async procedure calls. Ultimately it arrives at the box that is the subject of your question.
The answer to the question that is consistent with the notion "All the way down" is that "I am an async method". What does this async method do? Well, that depends on what you want to do? If you're reading or writing a file then it's an async call to read or write a file. Are you making a database query? Then the call is to an async method that makes that query. With this in mind I guess you can say that often what is at the bottom will be a device driver method, that performs an IO access asynchronously. Although it could easily be a long running compute bound asynchronous operation that you wish to perform such as processing an image, or video file.
It's worthwhile noting the right hand side of the call graph here too. Although often you may want to call async methods all the way down, the right hand branch of this call graph shows that you don't necessarily need to call an async method at the bottom at all.
