Ground-level enhancements (GLEs) comprise the high-energy end of solar energetic particle (SEP) events and constitute a special class of events in which ions are accelerated to relativistic energies, causing a significant sudden increase of solar cosmic rays at ground level, as detected by e.g. neutron monitors (NMs). Given their extremity, these high energy SEP events are also being recorded by spacecraft in the space covering a wide energy range (up to a few GeV). Onsets of these events are related to particle acceleration, while the role of the interplanetary transport is considered to be minimal (scatter-free propagation), thereby relativistic SEPs (i.e. GLEs) are good candidates to unfold long standing issues on the particle acceleration at the Sun and to pinpoint their parent solar drivers – a topic of significant interest to the heliophysics community. A considerable progress in both observations and modeling of such events has been achieved in recent years  (e.g. Bazilevskaya et al., 2013; Bindi, 2015; Whitman et al., 2017). However, GLE events are rare (only 72 events have been registered since 1942, see http://gle.oulu.fi), and their coherent analysis is not a trivial task. Thus we are still unable to clearly identify the parent solar drivers of the high-energy SEP events and, hence, mechanisms of their acceleration and propagation are not fully understood yet. In addition, recent independent studies by Kuehl et al., 2017 and Vainio et al., 2017 have shown that there is a significant number (>25) of high-energy SEP events recorded by spacecraft at energies >500 MeV with no indication that these particles reach the Earth’s surface (i.e. NMs), albeit the measurements of NMs situated at high-altitude polar sites show some signal for a few events (Poluianov et al., 2018). This has lead to the definition of a new class of SEP events, the so-called sub-GLEs (Atwell et al., 2015; Mishev et al., 2017). Evidently, the existence of such high-energy SEP events poses a challenge and creates new opportunities for a breakthrough in our understanding of energetic particle acceleration. Here, we propose an ISSI international team of experts that will identify a number of such events to be analyzed in detail, employing both remote and in-situ observations. We will bring together, particle measurements from the ground (NMs), and the inner heliosphere (Solar and Heliospheric Observatory – SOHO, Solar and Terrestrial Relations Observatory – STEREO, Advanced Composition Explorer – ACE, Geostationary Operational Environmental Satellite  – GOES, Alpha Magnetic Spectrometer – AMS-02, Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics — PAMELA), which will be further inter-calibrated. Comparisons will be made with the modeling outputs and the current theories on the particle acceleration will be tested against our findings.